Just to get it out there, as I have not found anything but unsolved threads online: After a lot of messing around, I finally could sync my contacts with OwnCloud over CalDAV. There is no official addon, Zindus and FunambolSync won't do, and SOGo Connector is outdated. What to do?
Well, simple - SOGo Connector is perfectly compatible with Thunderbird 13.0.1, it seems. It's just the version number in the addon that differs. I do know it's risky, but I just tried changing that. To do so, get SOGo Connector 10.0.2 from http://www.sogo.nu/english/downloads/frontends.html
. Then save the file (don't install it! - right click, save as), rename the .xpi to .zip and open the archive. Open install.rdf in the root directory and look for the string MaxVersion. On the same line, it says 10.* . Well I just changed that to 15.*, re-saved the archive, went to Thunderbird and installed.
And well, it works
Open the Address Books dialog, File -> New -> Remote Adress Book, and enter a name and your CardDAV URL. Right click the new address book, synchronize. There everybody is
I tried downloading my contacts, changing a field in one, adding a test contact and deleting it again. No complaints at all, OwnCloud's GUI tells me every step checked out.
DISCLAIMER: This is VERY hacky, and it can potentially pose grave danger to your contact list. Use at your own risk. But if the developer of the plugin didn't go overboard and messed with anything but their own remote address book, I hope this will not affect the environment. So keep a backup of your CardDAV database, just in case, and hope for the best.
(Also, I know this is banal. But noone seems to have tried this so far, everybody just says support for SOGo is broken.)