I experienced this same problem with Debian 11 with GNOME 3.38.5: I had tried to change the Settings > Accessibility > Zoom Options > Magnifier Position from 'Magnifier cursor moves with contents'. The moment I selected the option 'Keep Magnifier Cursor centred', I was logged out & the login screen appeared... & whenever I entered the password for that account, the screen went black & the login screen re-appeared. The accounts of other users, also using GNOME, were completely unaffected.
I resolved the problem by logging in as another user & removing the file /home/(username of corrupted account)/.config/dconf/user & logging out. On entering the password on the login screen for (username of corrupted account), that file was re-created & it was again possible to login to that account. I'm not sure what caused this or whether it is relevant to your problem, though. Regards, Ken
