I just looked into the log files, and found out that people got blocked for trying to open old MP3 files from the jukebox.
The jukebox is supposed to handle all requests for MP3 files on Brain Tags. If the requested file exists and the time-out period has not passed yet, the file is served; in all other occasions a list of currently available files is presented.
However, after forwarding MP3 requests to the jukebox, all requests for old MP3 files got redirected to my spamtrap, effectively blocking visitors looking for MP3 files. Of course, I immediately fixed the problem.