If this can save you the headache...
We had tickets created in a queue that was marked as "Invalid". While none of the PostMaster filters, generic agent, mailboxes, services were showing an affectation to this queue.
We found out that if you have a mailbox assigning tickets to a specific queue and that you change that queue to invalid, the ID of the invalid queue is kept as the value in the database but the text displayed for the queue in the dropdown of the mailbox configuration display the first queue in that dropdown...
Great behaviour!!
Tickets assigned to invalid queue
Moderator: crythias