I keep on randomly encountering this issue with our users. One time, I did have to delete the user account. That obviously managed the issue but eventually it resurged, and we’re back to the same issue for some of our users.
We looked at the password, and email, and all seem to be correct. Any other areas that we have to look at?
Is there a place in your app where users are being created with a possible whitespace or something? Seems like it definitely must be an error in your app as opposed to an error with their user management system. Before deleting the users with an error make sure to check all of the fields closely for small differences.