in 7.5 when you opt to FREEZE the first instance of the Motif XS VST what options are you offered at that time and what did you select to do?

I click on the snowflake icon on the MOTIF XS VST instrument in the rack.
"Freeze Instrument Options" dialog opens
It offers "Freeze Instrument Only" or "Freeze Instrument and Channels" radio buttons. "Freeze Instrument Only" is selected by default, and I leave it like that.
It also offers "Tail Size", set bey default to 5%, which I also leave.
Finally a checkbox, "Unload Instrument when Frozen", is not checked by default, and I leave it unchecked.
After clicking OK, another dialog opens: "External plug-in is used. Freezing will be done in realtime", which seems self-explanatory enough, so I click OK.
A progress dialog opens, the piece plays back in real time (I don't hear it, but I see the project position marker scroll through the piece as normal pace), the progress bar advances to completion.
Upon completion, the dialog box closes, and so does the XS Editor.
The MIDI tracks all have lock icons on them, and the Editor icons are disabled.

When I asked you to SAVE the first instance of the XS VST, when you reopened the file, (before attempting to setup the second instance) does the Freeze Audio playback as you expected?

Immediately after freezing, I hit play, and the piece plays back, I can hear it. I can see in TotalMix that the only signla is the software signal from Cubase - the Moti is not being fed any MIDI to generate tones.
I close the project, quit Cubase, re-start Cubase, re-open the project
The project re-opens exactly as it close - track frozen, XS Editor closed. I see on the Motif LCD screen that it's being sync'd.
Hit play, plays back just the same - audible, no MIDI

As a test I tried selecting "Freeze Instrument and Channels" in the Freeze dialog. It may have made some other differences to what got locked, but in terms of our issue, I got the same result when I tried to load a second instance of MOTIF XS VST - Port Open Error.

I will read, re-read, and digest the info about he MIDI ports.

As always, thanks.

As a test I tried selecting "Freeze Instrument and Channels" in the Freeze dialog. It may have made some other differences to what got locked, but in terms of our issue, I got the same result when I tried to load a second instance of MOTIF XS VST - Port Open Error.

I was thinking to test the other option... to "Unload Instrument when Frozen".

I was thinking to test the other option... to "Unload Instrument when Frozen".

Let the people rejoice - success!

That was it. I could see no difference in the Freeze process, but when I next loaded a second instance of MOTIF XS VST, it went straight to syncing, and completed successfully with all the usual connections.

I have since created additional tracks using the second instance, saved with second instance open, closed, re-opened. No problems.

Freeze instance two, save, close re-open - perfect.

Going for broke - Load a THIRD instance, more tracks, save, close, reopen - all good.

Got cocky and tried leaving more than one instance un-frozen - nope, can't do that. Must have only one instance active at a time. No problem, we can do that.

Thank you, Mr. M., et al.

Tomorrow I start working on routing options to optimize workflow with this new miracle.

Thanks again.

Okay, I know you must think I'm hard-headed, but these things have a method to the madness. By unloading the instrument when you FREEZE, you make the ports available for the next instance. You can only have one set of 16 MIDI Tracks active, because you only have the one physical XS.

Thank you for being diligent in your efforts to work through this. I was convinced it would work, we just needed to run the checklist.

Jaw dropping attention to detail and follow-through. Hat off to OP and BM.