New users can't login

[Sunshadow99]

I'm running two older versions of Dotproject (2.1.6 & 2.2.0) and have the problem that new created users aren't able to login (didn't add new users for a long time...). The login page always shows the message "Login Failed". Existing users are working fine. The new users can be found in the database, there must be something wrong with the login process. Any idea how to solve the problem?