Reported by @Maxlath at https://fr.wikipedia.org/w/index.php?title=Sp%C3%A9cial:Connexion&returnto=Wikip%C3%A9dia%3AAccueil+principal or at https://fr.wikipedia.org/wiki/Special:CentralLogin/complete?token=....
When an account is renamed, this error message could occur:
It's HTML escaped, but should be printed raw.
Formatting happens in core AuthenticationResponse::newFail method.