okay i checked on of the groups i belong to, and discovered it was under the category "music". the group in question is the 'order of reason' http://order-of-reason.minitokyo.net/ , and i reckon it's more of a philosophic group than a music group. maybe the category philosophy (or something close) doesn't exist.
anyway then i checked the groups page in the top bar, and there are a lot (and i mean a lot) of groups under the
category music (such as hispanime, which is more of an hispanic group than a latino-music-lovers group).
that's it, i know it's not an important bug though XP
PS i know i've already complained about that, but the group doujinshi support club hasn't been working since MT3 was up http://doujinshi-support-club.minitokyo.net/