Reseller can't login

[smartcall]

Hello to all,

I have this strange problem:
One of the resellers can't login to his account anymore. He used to be able to do that, but all the sudden he can't.
I changed his username and password, and the problem persists. I also tried to login with the new username and password, and ISPConfig rejects the credentials.
What could be wrong?

P.S. Maybe important: this guy has two reseller account and somehow the usernames for both were the same.