You were using Gnome with X sessions? Be aware that with the upgrade to Gnome 49 gdm no longer supports X sessions, so you have to switch to Wayland or use a different display manager; sddm should still support X sessions AFAIK.
Or you might be affected by bug 1250513, see for instance after-update-no-login-gdm-could-not-obtain-user-info-gdm-greeter
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