Thank you guys for the report and good job on the workaround! Signing off should be enough to get it working.
It seems like Spotify killed the authentication flow via Login5 when using stored credentials produced by the AP with an access token with a different client ID than the desktop one.
This seems a structural and definitive change so I am afraid that this functionality will need removing, but it’s worth investigating when I have some time. It might be something stupid like an outdated version string somewhere.
Official tracking: Spotify token authentication fails with INVALID_CREDENTIALS on go-librespot 0.7.1 · Issue #364 · devgianlu/go-librespot · GitHub