Following your link I found https://github.com/owncloud/calendar/issues/1002 and the second comment has a solution that fixes the problem for me. I did a vimdiff on his file and without understanding much I can tell you that the changes look reasonable. I have only been testing for 5 minutes but so far it works.
Sync errors with Citadel groupware server calendar
I have been running the Citadel groupware server from www.citadel.org for years. Recently I needed a calendar to share with others, and as it claims to support that as well as ical/caldav I figured I use the same system. The Calendar works fine from Thunderbird. DavDroid seems to be able to sync changes from my phone up to the calendar, but not entries from the server down to the phone. So currently I end up using 2 products, iCal Import/Export 3.1 to sync down entries (if I try and use it to upload entries it seems to wipe the server’s calendar), and DavDroid to sync changes back up. I’d of course like to get down to one product, and DavDroid seems more simple in the UI if I could get it fully functional.
Citadel is a bit wonky on versioning now. I’m running on Scientific Linux 6.8 (RHEL derivative) and Citadel 907. I have DavDroid 1.6.2-ose from F-Droid repo on a Samsung Note 5 phone running Android 7 Security patch level May 1,2017 kernel version 3.10.61-10809541. Debug info here:
The server sends an 500 Internal server error on a CalDAV request. Please contact server support.