Indeed, the reason was a postmaster filter. I wonder, why it worked anyway ...
And I think this is potentially a very basic problem:
Postmaster filters are executed for every incoming mail on a postmaster account. Say we have a postmaster filter1 for "to: user1" and filter2 for "to: user2", each setting X-OTRS-Queue to queue1 resp. queue2.
This works if writing a request to user1 or user2.
What if you write to user1, user2? Both filters are executed, filter1 redirects to queue1, filter2 redirects to queue2. So only one ticket is created, in which queue depends on the filter settings and order.
How do you handle that cases? Would it be better to use generic agents?