File "/usr/lib/python2.7/dist-packages/txdav/caldav/datastore/scheduling/ischedule/utils.py", line 20, in <module> from twisted.names import dns ImportError: No module named names
It seems your installation of Twisted is incomplete, and is installing the twisted.names package. This is odd, because unless you've specifically installed Twisted from the sub-package distributions to exclude things, it would include all of tis subpackages.