@SimpleSmarts I just noticed that the release notes Volumio Changelog contain this
Update socket.io to 2.3.0
And looking at the original library (socketIO-client) which was used, wasn’t upgraded since 2016, I guessed there might have been a change in the protocol. That’s how I then found the “new” library (python-socketio) which is maintained. First test didn’t work, but then I saw a table on that new library which indicated which version of the library to use with which version of the javascript part, as volumio uses 2.3.0 (don’t ask my why they use an old, outdated version of this), I used the matching version.