Hell yeah i fixed it.
After Hours of comparing files between my productive Installation and the test Installation i found the following:
My ~/etc/check_mk/conf.d/wato/contacts.mk looked like this on the productive Installation:
contacts.update({'username': {'alias': 'username', 'notification_period': '24X7', 'service_notification_options': 'wucr', 'notifications_enabled': True, 'contactgroups': [''], 'disable_notifications': {}, 'notification_method': ('flexible', [{'parameters': [], 'disabled': False, 'service_events': ['w', 'u', 'c', 'r', 'f'], 'timeperiod': '24X7', 'host_events': ['d', 'u', 'r', 'f']}]), 'pager': '', 'email': 'username@example.com', 'host_notification_options': 'dur', 'force_authuser_webservice': True, 'user_scheme_serial': 1, 'fallback_contact': False, 'notification_rules': []}})
After deleting the user and recreating it it looked like this:
contacts.update({'username': {'alias': 'username', 'email': 'username@example.com', 'pager': '', 'contactgroups': ['everything'], 'fallback_contact': False, 'disable_notifications': {}, 'user_scheme_serial': 1}})
So it was definitly some legacy configuration which caused the issue in my case.
Hope that helps someone else figure out that issue.