It is a webpage. You can click Ctrl+Shift+I to open the dev tools and go to Console, see if any error appears.

Or run the backend manually through the Command Prompt:

cd C:\Program Files (x86)\Steam\steamapps\common\xVASynth\

resources\app\cpython_cpu\server.exe

Thanks for you fast reply.
The server.exe says: File not found error: [Errno 2] No such file or directory:'./resources/app/javascript/script.js'
[5676] Failed to execute script server
when I start the script.js : Syntaxerror Code: 800A03EA

The server.exe should not be searching for any .js files. 🤔

Does the script.js file exist at that filepath? Relative to the .exe file. Could try re-validating Steam files if it's missing. (Also, I don't get notified of messages here. If I don't reply, try sending a message on nexus or discord)

Got same issue, how to fix? ffs, re-validating Steam files didnt help

Still having the same issue, can't use the software

You can check the server.log file for the actual error. If it is anything relating to "socket", then some have fixed it by using the Nexus Mods version of xVASynth.