Unsupported version control system "darcs": Can't find an appropriate component, maybe the corresponding plugin was not enabled?
To solve it I grabbed the latest darcs trac plugin:
darcs get http://darcs.arstecnica.it/trac-darcsand installed it:
sudo python setup.py install
I then had to upgrade and resync each repository:
trac-admin /var/lib/trac/myrepo/ upgrade
trac-admin /var/lib/trac/myrepo/ resync