Hard to say, but could well be authentication. That username looks like it is using some non-basic form of authentication, and the plugin only supports Basic Auth - see XmlRpcPlugin documentation.
Problem with the version of plugin you used was that on any 'big' error like authentication, the plugin would raise a regular Trac error - which in turn makes a nice HTML page of information. That result cannot be parsed by the xmlrpclib and it complains of about junk in result.
In [6069] I re-did the error handling, so that most classes of errors should now return in the format requested by the caller. Please update to latest version of plugin from trunk. Also, turn on Trac debug logging - the plugin writes much useful information to the logs.
Please reopen if you think this is a problem with the plugin, and include the 'About Trac' list of software versions + any other relevant information.
