The only clue I could find from the system admin was that he recently installed the crappy SharePoint stuff from M$.
After hours of searching on the internet, I finally found the solution to my problem here: http://objectmix.com/inetserver/285116-http-error-403-2-forbidden-read-access-denied-2.html#post1008979
Basically what happened was that the SharePoint installer screwed up some hidden metabase property named AccessFlags. On my server it was changed to 30215. I needed to change it back to 519 for it to work again.What can I say? M$ sucks. As always!