I didn't have my login credentials available on a laptop, so I posted an answer as guest using a different mail address than what I used to register at Stack Exchange.
Later I decided to reset my password and wanted to log-in with my account to a different Stack Exchange community. However, instead of a log-in button, a "join this community" button was shown, which redirected to the sign-up page. Logging into a different community did not work either: after reloading, I was again "logged in" as the guest.
Deleting all Stack Exchange cookies finally solved the problem.