I think you are rigth about the transition items that are still left in the databaase.
Because there are a lot of those records in the database I think the best thing is to make a clean start again. Because the user and forum information in the database is not corrpted I will use Tool > Export and Import to get the relevant data in a fresh built system.
It is a strange problem that all of the sudden popped up and a fresh start is likley the quickest way to a solution.
When that does solve the bug I will contact you.
Best regards and thanks for your reaction,