Using DAVdroid 126.96.36.199-ose from F-Droid with ownCloud 9.1.4 (stable). One of the ownCloud calendars is shared with an other user but just read-only. That user is using that shared read-only calendar on an Android 7.0 phone using DAVdroid. DAVdroid recognized this calendar correctly as read-only by displaying the correct symbol on the account.
User do have two cards for one person on two different calendars (linked to each other). One of the calendars is read-write, the other one is the read-only calendar described above.
After user called a number from this card, some data should be written to the card (see issue 1290). My guess would be that this should be written just to the card of the read-write calendar, but DAVdroid also tries to write it to the read-only calendar.
So with every sync, users gets an error message mentioning DAVdroid can’t write to the read-only calendar.