No, have not heard this before. But incorrect routing can be the cause of odd behavior.

Not sure why you are using the UR22 for interfacing the MOXF, which is its own interface. Let's start there.

I thought that using a different audio driver than the moxf would easy the cpu burden. If fact, I have not selected the UR22 in any part of the process. I started out by selecting the "Yamaha moxf with vst editor" on Cubase and moving on regularly after that. As I see in the settings, only place where I can see the UR22 active is in the screenshot I attach here.

You have everything selected in the "In ALL MIDI INPUTS" column... You should only mark those devices that are actual input sources for MIDI Tracks in Cubase - these might include:

The "Steinberg UR22-1" - if and only if you have a MIDI device connected to its 5-pin MIDI IN jack.
The "Yamaha MOXF6/MOXF8-1" - if you connect the MOXF to your computer via USB cable
The "Yamaha MOXF6/MOXF8-3" - if and only if you have another device connected to the MOXF's MIDI IN jack (its data will pass THRU the MOXF and arrive in cubase as port 3

Never, ever, have Ports 2, 4 or 5 marked in this column. Please unmark those boxes in the "In ALL MIDI INPUTS" column"

The MOXF can act as its own AUDIO Interface and can act as an audio interface for any mic, guitar, keyboard, etc, connected to the A/D INPUT on the MOXF back panel. It can act as its own MIDI Interface and can act as a MIDI interface for any MIDI device connected IN/OUT to the 5-pin MIDI jacks on the MOXF back panel.... This requires that the MIDI IN/OUT parameter is set to USB.

USB for the MOXF is a 5 port connection (the equivalent of 10 MIDI cables). Each PORT represents a MIDI IN and a MIDI OUT.

The MOXF itself uses PORT 1
Port 2 is for Remote Controlling your DAW
Port 3 is for one external MIDI Device connect to the 5-pin MIDI jacks
Port 4 is reserved for the Yamaha MOXF6/MOXF8 Editor Standalone/VST
Port 5 is for the REMOTE EDITOR (that allows you to build controls for various VSTi plugin synths you may want to run from Cubase - it includes room for some 50 VSTi templates.

the Yamaha Steinberg USB Driver wine you select the MOXF gives you 4Channel (Dual Stereo) operation for audio from the MOXF. The A/D input uses USB 1/2 and the internal MOXF uses USB3/4 - although each of the 16 MOXF synth PARTS can be redirected, as necessary to USB1/2 when you wish to record a synth PART to its own audio track (isolation).

With this arrangement you can play and sign and record each to separate audio tracks.
or you can record each PART of the MOXF to its own separate Audio Track (in multiple passes) by isolating the track in question to its own isolated Stereo output

Cubase allows you to switch devices whenever you require, so say you want to record using a CONDENSER Microphone (The UR22 will provide the +48Volt Phantom power necessary to make it run... Conveniently the UR22 uses the same driver as the MOXF - you can select which device is being used.

Is the setup in the pic attached ok? Should I uncheck nr. 2 from the column "visible" as well?

Is the setup in the pic attached ok? Should I uncheck nr. 2 from the column "visible" as well?

No, I usually keep them visible but visibility has no effect on what is sent to your MIDI Tracks. I only mentioned the critical "IN ALL MIDI INPUTS" column. Just so you know WHY... This allows you to use several different controllers to feed data to your MIDI track. Literally, IN ALL MIDI INPUTS allows you to select on which PORTS incoming MIDI data is routed to your tracks.

MOXF6/MOXF8-PORT 1, as I explained, is the Port used by the MOXF itself for the internal synth engine
MOXF6/MOXF8-PORT 3 is the 5-pin MIDI port of the MOXF's back panel - (say you had a non-weighted action keyboard that you would rather use when playing organ sounds, you could play on that as opposed to playing on the MOXF8 weighted action - in fact both keyboards are now eligible to input data to your MIDI Tracks. (Basically, this is how you define what is an Input Device you want to give permission to write data to your MIDI Tracks. Typically, each Midi Track is set to a specific device... With this setup you are able to define multiple devices as inputs, then simply set the Track Inspector's Midi In = "All MIDI Input"

If you have something else, another device capable of acting as Midi controller - perhaps a Yamaha DTX M12 (MultiPad) drum controller, you could plug its 5-pin MIDI OUT to the UR22's MIDI IN, and in a similar fashion it would pass that information THRU to your MIDI Track allowing some one to play along with you and you could record all the devices.

You DO NOT want Yamaha MOXF6/MOXF8 PORT 2 (because it is reserved as your REMOTE CONTROL (DAW CONTROL) Port. If you mark this port touching front panel buttons on the MOXF may trigger notes on the tone engine that is connected to that PORT. The MOXF has the ability to act as a control surface for DAWs... So when DAW Remote mode is activated, the front panel buttons and knobs send control messages to the selected DAW, allowing you switch screens, arm tracks, mute and solo tracks, start and stop, etc., functions in your DAW software (templates are made for Cubase, Logic Pro, Digital Performer, Sonar).

You DO NOT want Yamaha MOXF6/MOXF8 PORT 5 because this port is used for the REMOTE EDIT.

In fact, PORT 4 disappears because it is "reserved" for the Yamaha MOXF6/MOXF8 Editor Standalone/VST which you have running! PORT 5 only activations if you are running the REMOTE (VSTi) EDITOR.

Highlight any MIDI Track... next to it's MIDI IN on the Track Inspector, you will see "ALL MIDI INPUTS" as the default input to that MIDI track. The column IN "'ALL MIDI INPUTS'" is what you define by marking those selected boxes... Making sense?

It does not matter what is "visible' or invisible.
Also just FYI, the ACTIVE / INACTIVE (STATE) is just that - a status report - it is not a setting, it is a STATUS REPORT about what is detected - so I can tell that you have no device connected to the UR22's MIDI In; nor have you activated MOXF8's REMOTE EDITOR... which is fine.

What about your original issue OVERLOAD?
Please find the VST PERFORMANCE meter - it is listed under DEVICES > VST PERFORMANCE
That will help us see what your CPU is doing with what you have active.

The indications of the VST PERFORMANCE meter while exporting were normal (low, at the left side of the meter). The problem still happened anyway...
Also (I don't know if this is relevant), if I click the "x" at the "visible" column next to port 2 it says that "the port is currently used" and that "hiding it will disconnect it". Why then I removed the "x" from "all midi inputs" at the same port? (rookie question I guess).

The indications of the VST PERFORMANCE meter while exporting were normal (low, at the left side of the meter). The problem still happened anyway...

Then I don't understand the problem. Are you receiving an error message? If so, which one specifically? What makes you say that there is a "CPU overload", I guess is my question.

Port 2 could be in use because the Extensions will automatically select Port 2 of the MOXF for the REMOTE DEVICE. The MOXF can be used as remote control surface for Cubase, Logic, DP, and Sonar.

You seem to have still both audio interfaces (the UR22 and the MOXF) in your setup. Have you tried if the problem still occurs if you start your PC without the UR22 connected?
It looks like the two audio interfaces are fighting for the clock master. 😉 If you use more than one audio interface, they have to be synchronized. One of them has to be the wordclock master and all others have to be in slave mode. If you want to use the Editor VSTi and record audio from your MOXF, you should somehow select the MOXF as audio clock master in Cubase. (I'm sure Bad Mister can explain how to do this in Cubase) If you want to use the UR22 additionally to your MOXF, it must be set to be an audio clock slave. Otherwise you'll have two drifting audio clocks which is pretty fatal in digital audio. But as Bad Mister already said, you don't really need the UR22 to record your MOXF. You can just connect your stereo or your headphones to the MOXF outputs to hear what's coming from the MOXF and from Cubase (that's what the white fader is for) and you could even plug in analog sources like a guitar, a mic or a CD player to the A/D inputs and record these.

Here is a screenshot of the error message. I get this after a different processing time each time. Generally appears before 10%. After that the bar turns to 100% (with no resulting file of course).
I tried disconnecting ur22 before opening cubase. Unfortunately the problem persisted (so that rules out the clock issue).
Is there a way to monitor the general behavior of my computer (something like the VST PERFORMANCE meter, but more general, a windows tool maybe)? Maybe this is a computer problem after all?

Okay, thank you for the screenshot. It helps us understand some things that may help.

It may well be a computer issue. But before we send you off in search of solutions elsewhere, let's try a few things "from scratch" that may straighten some things out. We see that you are using one of the preset templates. Which is not a bad thing, but I would rather you setup manually. First, because it will not reserve as many resources right out of the gate and this may tell us where (at what point the CPU runs into an issue. And second, we may avoid the issue entirely by setting up one step at a time. Or third, it could just be how you set the EXPORT > Audio Mixdown setup.

So what I'd have you do is start a new Project but do not use one of the Templates made for the MOXF, we'll manually go through the setup process.
In doing so you get to see how we are routing signals through the process.

Put the MOXF in SONG mode - select a blank SONG
Put the MOXF in "DAW REC" mode
You do so by holding [QUICK SETUP] + [A/1] buttons together
"DAW Rec is applied"

Launch Cubase
Select the "EMPTY" Template (rather than one that sets up the MOXF VSTi - we'll do that manually)

On the right side of the main empty Project window select "RACK INSTRUMENTS"
Find the MOXF VST under the heading "External"
When you select it this will launch the MOXF VST and place it in the VST INSTRUMENT folder in the Track Inspector area.
You will be asked to create a MIDI Track into which you can record - create that Midi Track.

You will now have a MOXF VST folder (as the first item in the VST INSTRUMENTS folder) with an Audio Lane, an Automation Lane, and then a MIDI Track assigned to the "MOXF6/MOXF8 VST - MidiIn"

The MOXF Editor VST interface will launch. This is your MOXF hardware integrated into the computer... Such that any changes you make on the MOXF they are shown in the computer, any changes you make to the Editor on the computer are immediately made in the hardware MOXF. When the EDITOR is ONLINE it is a 1-to-1 relationship with the hardware:software.

As you play you should see two lanes responding to your key presses. One is MIDI data; the other is audio.
The audio lane should show a stereo (left/right) meter that responds to audio being send out by the MOXF via USB
The MIDI track also has a meter... It is a single bar meter because this is Midi data (velocity and controller movements, not sound) Midi data is not stereo or mono, it is not sound at all.

So when you press a key on the hardware MOXF, there is MIDI data generated that travels OUT via USB to the currently selected MIDI Track in Cubase, this MIDI track is set to "All Midi Inputs" and is set to go Out on Midi CH 1 and is set to go specifically to the MOXF VST Midi In. (The Editor)

As you play on the MOXF keys you will see the keys of the editor respond, this is because MIDI data is routed by the currently selected Midi track in Cubase to the VST Editor's MIDI IN.
Next the EDITOR which is ONLINE, communicates with the MOXF tone engine. The tone engine receives Midi messages on Channel 1. Part 1 will respond... In fact all Parts you set to channel 1 will respond.

NEXT: The MOXF generates audio in response to the Midi messages coming in via USB.
This audio is routed OUT of the MOXF to two places: one is directly Out to the main Left and Right output jacks.
The other is through Cubase via USB before finally going back again to the MOXF as audio interface > to the outputs.

These two paths are referred to as "Direct Monitoring" which is audio going directly from the synth to its audio outputs. And the path that travel to the computer to be time stamped/recorded to the hard drive, then sent back to the audio interface (MOXF) - this is referred to as the latent signal.

You can select to monitor yourself "direct". If you do so, then you will want to "Mute" the latent audio signal in the VST Audio Lane.

Each time you want to record a new musical part, simple Add another MIDI Track assigned to the "MOXF VST MidiIn". Each new track will automatically increment its MIDI OUT channel.

When you select that new track, the Editor will select Part 2 automatically. The MOXF VST / Cubase / MOXF are all synchronized so that when you select a Track the corresponding Part in the VST Editor is selected and vice versa.

Setting up this way... Now try exporting audio mixdown.
If you are still having issues, please show us the Export Audio Mixdown dialog box so we can see what task or tasks you are having your computer do on this Export.

I followed through, problem persisted.
I attach you my export settings window.
I noticed something new with this procedure though:
There was a glitch sound coming from the left speaker, like a couple of sixteenth notes, reappearing irregularly every app. 2 to 30 seconds. I also took a screenshot of it and I attach it to you.
Dionysis.

P.S. I had task manager open while exporting to see what happens. In the pic attached, the cpu overload occurred (2 times in this session) at the graph drops.

Dionysis wrote:

I followed through, problem persisted.
I attach you my export settings window.
I noticed something new with this procedure though:
There was a glitch sound coming from the left speaker, like a couple of sixteenth notes, reappearing irregularly every app. 2 to 30 seconds. I also took a screenshot of it and I attach it to you.
Dionysis.

Okay, try a different Export setup. We are not sure what you intend to do with the exported Waveform. But you are set to create a 44.1kHz 32-bit floating point stereo wave. (? Is that what you require?) try a smaller bit depth as an initial test.

Try setting the "IMPORT TO PROJECT" area so that the audio is accessible by this Project. Mark the "POOL" and "AUDIO TRACK" boxes. This will allow the export function to place your waveform so you can play it back in the Project.

In your screenshot "glitch.png" please show the MOXF Editor VST (it should have opened when you launched the MOXF VST. Also go ahead and record something. Create another track and record something else... Then attempt to EXPORT > AUDIO MIXDOWN to a stereo .wav you can export and play on your computer or load to the MOXF. 44.1kHz, 16-bit. Make sure you mark POOL and AUDIO TRACK.

Different export settings didn't solve the problem.
I attach you the new glitch screenshot per your request.