It couldn't start a torrent. The error was "path xxxxxx disallowed due to security reasons." The path seemed to be a file in the torrent package, starting with ._.
The error was solved by editing /usr/lib/python2.4/site-packages/BitTornado/BT1/btformats.py:
reg = compile(r'^[^/\\.~][^/\\]*$')
reg = compile(r'^[^/\\~][^/\\]*$')
The dot was removed.
The regular expression seems to be used to disallow any file name starting with /, \, ., or ~. That's understandable. By removing the dot it would allow files starting with dot. I'll just add it back after I'm done with this particular torrent.