"It also sort of sounds like you have more than one version of osTicket
installed and collecting emails. Which would also explain why you
cannot see the tickets in the live one."This is exactly what was the case. It is really embarrassing, but the problem did not have anything to do with OSTicket itself. When we moved to another server some time ago the OSTicket files remained on the old server. Although they were not accessible from the internet, there was a cronjob still running on the server which pulled the mails from the mailbox. We managed to shut it down so now hopefully this has an end.I'm really sorry i wasted your time. Thanks for trying to help me.