I'm trying to remember once... I had trouble on a server way back when. They enabled some security lock down, and I couldn't post anything until I turned the attachments off.
But it doesn't sound related.
You said you tried with a clean addpost.php script. Did you also try with the appropriate addpost_preview.tmpl (I think it's called).
If you've done any hacking, my guess might be a misaligned </form> tag.
But this does sound peculiar. I can't think of what it could be.
