The missing posts are neither started from a deleted member nor really old posts. The latest answers are from april or may, that's not very old for me. It seems almost randomly for me which post is displayed and which not. I saw that some posts are missing while I was looking for 'sticky' posts - some were there and some not (not only sticky posts, after I saw that some of them are missing I've looked through the board and found some more missing posts).
I'll have a look if deleted users have answered in the missing posts, maybe this is something unique. But on the other hand I saw posts written by deleted members and they were displayed correctly. Sorry but I have no clear answer :/
