Hmmmm....
That might be a database issue.
I'm not sure.
You could try closing the forums, then do the optimize command from the Admin menu... see if that helps.
If not, you might need to run the SQL command REPAIR TABLE w3t_Posts and perhaps w3t_Users
I won't pretend I know. If it were me... I'd probably try it.... but I would always backup first.
But since you can see the posts, and they aren't appearing... I'm thinking it's a database thing that somehow the delete went wrong and the usernumbers are confused.
Maybe a bug... I don't know.
I'll defer.... maybe one of the Daves or Rick will have an idea.