I love Movable Type 's great error messages…

means that you used the tag "MTEntryCategory" in the archive files configuration instead of the "MTArchiveCategory" tag – which of course should return the same result… but because of some dumb perl coding you have to use the first for the individual archives and the latter for the Category archives…

oh well… all I found on the net about this problem is that people didn't find the mistake and re-setup everything from scratch… puhhh… what a mess. Hope this entry helps someone out there to save him the hour I just spent figuring this out.