Fixed it at my board:
Find this in ManageAttachments.php and RemoveTopic.php
removeAttachments('a.attachmentType = 0
and replace it with
removeAttachments('a.attachmentType = \'0\'
After done this, you should be able to delete topics again.
Now execute this: Administration-Panel -> Forum Maintenance -> Recount all forum totals and statistics
This should fix the statistics and after deleting a topic the rest should be visible.
Maybe sbdy can change the subject of this post to "Problems with deleting Topics" or something.
Greets,
Samurai