I suspect it's due to the URL being seen as a long string. You'll notice that here, vBulletin (or a modification of it) can make urls intelligent by adding an ellipsis to long URLs.
I believe this is a common problem with phpBB; you can try this modification, but I don't know if it's applicable to your version number or what. Otherwise do a search on their community forums. (This is under the assumption that you are indeed running phpBB, which it would appear you are.)
EDIT & Off-topic: By the way, I really like your site. System 7 is one of my favorite OS's. (Just behind A/UX & Rhapsody )
It is indeed phpBB, with a super-modified theme that is entirely CSS free, and a fairly recent version of the phpBB software (like early June). Thanks for that link I shall try that. Thanks for the comment on the site too I always appreciate it!