A friend of mine had the same problem yesterday using Opera. We could avoid it when switching "Tracker blockieren" (block trackers) in Preferences to "Off". Looks like google changes something on their pages (related to the support of login using google account on BGA?) that breaks many users who have any add blocker or tracking blocker active, even if they use an BGA-only account and not the "login via google account" feature.
It was reproducible before your workaround was implemented. The message already happend in private window before any login data was insered into the fields, meaning it is independent of the individual user accounts.