Due to the amount of spam that was recently happening on the forums, we increased the registration security a little. Also, because of the huge amounts of fake spam-users, we did a serious cleaning in the user tables.
Basically, if you had 0 posts, and wasn't part of any group (to shield non-posting Supporters), your account has probably been deleted -- feel free to re-register though!
On the up side, I don't know if it's just me, but the forum seems to be working much more smoothly now :).