If the username is identical with the email, the login will fail.
I installed mailusername afterwards and this error occurs because some people simply chose their mail address for the username field.
The erasure of the usernames in this cases resolved the problems.
Oh, excuse me, this does not seem to be the right solution, it is a little bit tricky in my Contao installation.
I set username=email manually in SQL manually and hope for improvement…
If the username is identical with the email, the login will fail.
I installed mailusername afterwards and this error occurs because some people simply chose their mail address for the username field. The erasure of the usernames in this cases resolved the problems.