There can be a variety of reasons why your GEMA credentials do not work on the login page. One of the reasons may be that the GEMA online access is not yet activated. To be able to use MusicHub, you have to apply for the activation.
The required form can be found here. Please fill it out and send it to mitgliederservice@gema.de.
If this does not solve your login problems, please contact our Customer Support Team and we will see how we can solve the problem.