I have gotten some mails about that it's not possible to post in the forum, and it's the spam filter (akismet) that has blocked some users. Not sure why, but it can be a blacklisted url/email or something. I have now gone through some posts and unmarked them as spam, and hopefully akismet will learn that these posts should not be blocked. I will monitor the spam inbox from now on to see if it's really spam or not.
The reason I enabled the spam filter is that one person posted hundreds of spam posts in old threads and messed up the forum bigtime. So it's good to have such a filter, but not if it blocks regular posts.
Btw, I can see that several users post test posts to see if it works, and then akismet will probably recognize them as spammers, and block their next posts too. This might be the problem.