Further on down in the file I get the following the help dump from
twistd and the following message: /usr/local/bin/twistd: Unknown
command: caldav
Are you *sure* you don't have Twisted installed? The traceback lines are quite clear on the fact that /usr/local/lib/python2.7/dist-packages/twisted is where your Twisted installation is.
This still shouldn't happen, but it would be good to have a correct idea of what you have installed and from where before diagnosing.
-g