When a user with a User Type where Report Center is enabled tries to access the Report Center, she is instead re-directed to the login screen of WorldServer. When the same user is associated with a different User Type - for instance Administrator - she can access the Report Center without problems.
Resolution
Additionally to the option Report Center, in order to be able to access the Report Center, the User Type belonging to the user needs to have the option Web Services enabled as well. With both permissions Report Center and Web Services enabled and saved in the User Type, the user will be able to access the Report Center.
Root Cause
The option Web Services is not enabled in the user's User Type.
