The error was between the keyboard and the chair, user tried to use his email address to login.
You owe me 50€c, I bet on this

So it seems there is a other bug in the way the user error is reported back cryptically instead of detecting the '@' sign and report, 'please use your username only',
I agree, error messages must be readable and understandable by final users
please open a NFR, thank you
or we allow full email addresses too (which is common out there).
quite useless, we're using real accounts and so having the @domain part would be useless..
I mean: we'd accept the email but we'd edit the code to consider just the username part.. moreover, often users have an email address which is just an alias..