Did you also set the path to Nvim in settings.json? I had to do so to clear at least one error.

Yup. I set it to /run/host/usr/bin/nvim after exposing system libraries and binaries to VS Codium through KDE's flatpak permission manager. Prior to that it kept throwing me ENOF errors (or something like that, I don't remember now).

Unfortunately, that "disconnected" error is either not caused by a race condition for me or I was really unlucky, because at some point I restarted the extension 30 or some times out of frustration and nothing changed 😅