This sounds even stranger then… I don’t use one, but still get intermittent 403 errors from Google Drive.
I had one for a short time. I guess I can set one and see whether I can make 403 errors worse to debug.
One known problem with the user interface password is that browsers used to autofill it into bad places.
This typically showed up on Restore though because GUI password got put in the encryption password.
2.0.5.112_canary_2021-01-20 put fix in Canary, and it should be in the 2.0.6.x Beta releases.
It’s possible that there are some other files to make autofill-proof, if that turns out to be issue.