how is your Inbox configured?
- Is it a simple mailbox with tenant status mapping or an Inbox with XSL Transformations?
-> if yes, did you check the QuickAction?
- Is it only (and always) happening to Problem ticket updates and new tickets are generated as they should?
-> If it is only happening to Problem tickets, have a closer look at the Integration Logs.
- Are you using HEAT On-Premise?
-> If yes, check the logfiles.
We fixed the issue by recreating mail setting of Problem tickets mailbox. This time we used OTHER instead of PROBLEM from object selection.
There is no explanation of why it was working with previous settings until it decided to stop working and how new setup is better than previous one.