User can no longer post

Hello.....I have recently set up a forum at http://avebury.forumn.org/index.htm One of the users found that he could no longer post messages or re-register. I deleted his account and allowed him to re-register but he is still finding that he can't post. All the postings he made when everything was working OK are now labelled "guest". He seems to be the only user with this problem.....Can anyone help please?