[otrs] ticket assignment by TicketID

hello I have upgraded OTRS to version 3.1x. When I write an email to the system with a valid TicketID in the body or the subject OTRS does not assign it anymore to the correct Ticket but makes a new one instead. An installation of OTRS 3.1x from scratch does correctly. It seems that I have a missing configuration option somewhere after the upgrade, but I am not able to find it. can someone give me a hint? thank you very much. cheers kuene