Basically, what you can try is alter the sandboxing behaviour.
First check your console after you launched contacts for lines like these:
Contacts(3186) allow mach-register com.apple.AddressBook.AddressBookApplicationFrameworkIPC
com.apple.Addres(3189) deny network-outbound /private/var/run/mDNSResponder
Failing after a number of tries.
If you don't see something like that don't follow the next steps. You will find more information that might help.
You will have to edit a basic system-file by following the guide here: https://gist.github.com/mems/57a34fcf89441ae96b0d
Please be carefull and know what you are doing. Having a backup is always a good idea. Altering systemfiles is not why we all have bought a mac. But it seems like Apple doesn't really care since this problem is with us since the betas of 10.11.
For now I somehow fixed the issue by altering the .htaccess file so the OS X Dialogue recognizes the path. It seems somewhat irrellevant what server path you put into the dialogue. When you check it later - it reads something different, than what you put in. (bug)
I'm on 10.11.6 and currently change from owncloud to nextcloud, but it's almost the same for now. iDevices sync fine as my old Macbook Pro on 10.8.5
The procedure with .htaccess and changing the systemfile made my sync somehow bearable. However it still looks not 100% reliable. Deleting and adding contacts works good, but updating not so much... Still checking that out.