@murgero EteSync is CardDAV compliant and SOGo is not. I've had trouble making SOGo connect with Apple Contacts on both macOS 14 and iOS 13. EteSync is working for me.
Initial setup of EteSync is more painful but:
- in the end it works
- you get properly encrypted data on their server, with no secret master keys with the provider (open source, we can check)
- European company (French I believe) so there is a hope they walk the talk about privacy (US companies by law are not allowed to provide privacy).
The price of admission is $24/year which is fine, as one doesn't have to run the server. There's an unencrypted at rest CardDAV subuser included which will allow people who care about privacy to share an address book or calendar with people who don't (looking at most of our girlfriends/partners here, although I'm sure there are some extremely privacy conscious ladies who are frustrated with their mainstream who-cares-about-privacy-i-don't-have-anything-to-hide partners).
EteSync is running successfully for me and syncing between macOS and iOS 13 right now but there's a lot more testing to do before I can be sure that it's as good as billed. In macOS Contacts the original groups even turn up and work as they should. In iOS Contacts there is only one group "My Contacts".
Some long term users feel very positive about EteSync. Aerion asked ProtonMail to include it:
EteSync provide a ready to use, open source, and well established solution for adding end-to-end encryption: Etebase (https://www.etebase.com/). It already works with Android, iOS, Windows, Linux, macOS, Thunderbird, Evolution, Kontact, Tasks.org, and any other CalDAV/CardDAV client.
I've been using it for a number of years already, and it's never failed, not once. It's the only sync solution I've ever used that's not had problems with duplicates or missing data.
Reliable sync with no data loss is one of the hardest of software tasks. EteSync for now managed to retain groups in iOS/macOS which none of the other CardDAV calendars I've tried have managed.