Interestingly, the problems I first saw a few days ago and considered analysed later, turned out to be of a different nature: Safari seems to be the culprit rather than Movable Type. As the problem occured at the same time that I experienced low disk space and other errors on our server, this was a tricky one and hard to see.
Interesting as well, that the same problem occured elsewhere pretty much at the same time. Probably because we were hit by the same spammer.
The Vig-Rx bastard spammer has hit my comments on two or three occasions as well. It usually picks old entries.
Received data seems to be invalid. The wanted file does probably not exist or the guys at last.fm changed something.