I don’t know whether this falls within the obligation of the operating system or okhttp. However, I wonder why you have an IPv6 address when your carrier doesn’t support IPv6?
Sorry for my late reply. Let me express clearly.
My server support both IPv4 and IPv6, and my Android phone only support IPv4. I added both A record and AAAA record for my domain.
I look debug.txt file, it seem DAVdroid only to connect IPv6.
Then I deleted AAAA record, and make DAVdroid try again. And it seem DAVdroid connect it, but SSL handshake fail.(I think I should buy a new phone, Android 4.4 seem not supoort TLS1.2)
Are other apps working?
Other apps works so far, like nextcloud-android, NextCloud SMS. GPSlogger I also meet SSL handshake fail situation.
 GPSlogger developer seem working on support TLS for Android 4.1: