Let’s try some simple things first. You are Signed In and have a good connection...
First, you cannot be in “Incognito” Mode

Try using Chrome without the “Incognito” mode ... your other WebMIDI app is not using “Incognito” (probably why it is working and Soundmondo is not)...

The SYNC button will be active once you have selected either a MONTAGE or MODX sound — found under “Browse”
Select the Item - then the SYNC button will activate.
let us know.

Hey BadMister,

I don't remember why I tried "incognito" mode, I guess I was just looking for a way to make it work.... regardless I've tried also without incognito mode and it still doesn't work. I tried "Initialize All Settings" and now soundmondo doesn't even mark the MODX as connected. Regardless, I can use the MODX as a midi controller in REAPER (with incredibly low latency I might add!).

I have also tried using my reface with the same cable and everything just refreshing the browser and it works.

Something interesting is that the MODX shows up as three devices: "2- MODX-1 , 2- MODX-2 , 2- MODX-3" In my DAW only "2- MODX-1" seems to be working. Also worth noting is that in the Web Midi Monitor there seems to be no traffic and when I connect either the Reface CP or my Casio Privia PX-5S there seems to be a whole lot of midi messages even if no note is pressed. Maybe that's something worth noting? Seems like the MODX shows as a connected device but there is definitely no midi message being received via webmidi.

Any ideas on what to do?

So... I don't know what I did but now the MODX is showing up again as connected and the sync button is available! I can even see the midi messages in midimonitor. So some success at last.

Unfortunately.... SoundMondo still doesn't work... After pressing SYNC even though the URL is that of the sound I just see the same stuff as when creating a new sound plus it seems like nothing happens on the MODX.

Something interesting is that the MODX shows up as three devices: "2- MODX-1 , 2- MODX-2 , 2- MODX-3" In my DAW only "2- MODX-1" seems to be working.

Correct. The MODX has three ports
MODX-1 is Port 1 — the main MIDI In/Out for Note-Ons, Controllers, Tempo (This is the USB port the MODX itself uses)
MODX-2 is Port 2 — unused (reserved for future features)
MODX-3 is Port 3 — the 5-pin MIDI In/Out jacks on the MODX back panel. The MODX can act as a MIDI interface for an external device when using the MODX via USB.

Your computer ‘thinks’ you have two MODX’s. The number “2” as a prefix means your computer thinks it is communicating with the second of two MODX’s. Not sure if that is at root of all your issues, but if you don’t have two — you might want to uninstall and reinstall your Yamaha Steinberg USB Driver... and reboot your computer.

Thanks Bad Mister!

After uninstalling and reinstalling Yamaha/Steinberg USB Driver, seems like i managed to successfully use Soundmondo with the MODX. As you can see in midimonitor the "2-" prefix is gone and now after pressing sync a MODX6 image pops up on top.

Excellent!

Half or more of the soundmondo issues with browsers is solved by uninstalling the drivers and reinstalling. This gets rid of the "#-" prefix to the driver which gets around a bug Soundmondo has. I see this as a bug because it affects so many users and does not match what other software does such as Cubase. So your instrument could be working fine with everything else - but Soundmondo (webmidi) has a "hard-coded" driver name that does not deal with the prefix.

If you really did have multiple Montages - I think the driver should cycle through all driver instances and pick the lowest number. This would be infinitely better than the current method of just not working. There are so many ways to get the "#-" prefix through proper usage. It's a huge "ding" on the system to have most Windows users run into this problem and some don't have the same access to support (perhaps self inflicted).

That said, I don't see any changes coming given this has been a known issue since at least 2018.