Found in Sentry:

Can't call method "auth_realm" on an undefined value at /home/musicbrainz/carton-local/lib/perl5/Catalyst/Plugin/Authentication.pm line 31.

When a user is renamed, the remember_login cookie still points to their old username until they log out and in again. As such, if their session has expired and we try to log them in with the remember_login cookie, we pass an undefined user to Catalyst and the log in attempt fails with the ISE above.