0xCAFEBABE is the usual first 4 bytes of a Java file.
<!DO in Latin encoding, which is, in all probability, the start of
It is therefore likely the start of a
404 error, or some other error page from the server
ie. You may be trying to load something that the server doesn't know. Check logs, try to load the jar manually, etc.
(BTW you might be better off embedding html code in a user macro rather than enabling the html tag, it's a lot safer.)