Hi Florian,
You need to be very sure that you actually login using the service account :-)
Also make sure the API service is started.
Did you test the API connection when you installed the Admin UI ? This is important.
Sybase has this KB about the issue: http://frontline.sybase.com/support/resolutionDetails.aspx?KBID=8494 with this solution:
Login as an Afaria service account to access Afaria administrator and/or add the user account in question to Afaria server role. If the user account is already part of the server role then re-save the User by deleting the User and adding the User again.
Mvh
Peter