I did a little bit of digging:
One of the things that changed on AlfaBB is that Simon had to upgrade the vBulletin software that runs AlfaBB because the old version was hackable (which actually was a PHP bug, not really a vBulletin bug, but because vBulletin uses PHP, vBulletin was exploitable).
Unfortunately, the newer vBulletin release seems to be less forgiving for problems that seem to exist in the Safari browser. Apparently, there are a lot of Mac users experiencing problems with vBulletin and other web sites. The majority of vBulletin developers and users seem to agree that it is a Safari problem, not a vBulletin problem (see
here and
here). But the results are inconsistent (see
here). The core of the problem seems to relate to the fact that "
Mac browsers consistantly refuse to allow CSS styling of form elements." Not pretty. Further proof that the problem is not specific to vBulletin can be found
here, where a script is available that uses different style sheets for Macs to circumvent some Safari problems.
So, what can you do then? The answer is simple and ugly: Try using another browser.
Firefox (the Mac version can be downloaded
here) and
Opera seem to be natural choices -- but they may have oddities of their own (as any browser has).
Hope this helps...