Assume you have a forum user with a username of: "firstname.lastname@example.org"
The user may login to the forum using his password.
However, if the user typed "email@example.com " (note the space following the email), the credentials wont' match.
The added space can be a result of copy/paste, or just an error from the user side.
I verified this behavior with this forum as well. It can be easily replicated by yourself.
I think this can help, and it may be a small change. Not sure however 🙂
All the best,