This problem cost me some time to figure out: I configured a new profile in outlook 2003 using credentials of a testuser to test something. Later i tried to start the exchange managment console (emc) and found all my permission where gone. Apparently the credentials from outlook 2003 conflict with emc credentials and emc uses the outlook 2003 credentials.
To fix this you have to remove the credentials from the credential manager found in the control panel. After that, emc works again.