Line 116 (r12025) of auth.py prints a log message, but has a typo since it only provides one string, but indicates two will print.
self.log.debug('Unable to find user %s listed in group: %s' % str(m))
This looks like a trivial bug, though I'm not clearly seeing which variable contains the user name. In the meantime I can remove a '%s' to make the bug go away.
A similar bug (new ticket?) shows up at lines 500-509. Data is stored to variable 'res' or an exception is thrown which prints a long message. Then execution can continue, but even if the exception was thrown, the variable is still accessed at 506.
The end results of each of these issues is that that Trac loads with an error, but the error looks like a bug in the plugin, not a configuration issue (which becomes visible when you dig through the logs).