Carddav URL change
After upgrading from GO 6.3 -> 6.4, it seems that the URLs of the addressbooks have changed. Reported by @ayers and confirmed by @roland.alton
6.3: https://fairkom.net/carddav/addressbooks/ras/fairkom-568/
6.4: https://fairkom.net/carddav/addressbooks/ras/568/
This makes sense, as URLs do not change if the name of an addressbook changes.
Solution: reload your addressbooks e.g. in your DavX app and re-sync.