Unable to register email address

I have an qt.io account that works flawlessly otherwise. I can log into Gerrit with it (it shows the correct user name in the settings), but adding my email address to Gerrit fails: clicking the verification link in the email sent by Gerrit fails with the error message: Identity (...) in use by another account (which is definitely wrong).

My email address will follow in a separate email (as suggested here)