Hello Luca,
There are some recent posts suggesting a similar problem - you may want to try the steps suggested by @Arlen, here and here, which appears to have worked for some other users.
Edit: if not, you may have to go the ‘long road’ - uninstalling and reinstalling the Roon server software from your SonicTransporter, and then (probably) having to reload a recent backup - if you have one - to get back to a state as close as possible to where it all went pear-shaped. It’s what I resorted to before noting these posts. So it seems worth trying the shortcut first - toggling local playback twice.