<s:message>Invalid Mimedir file. Line starting at 12 did not follow iCalendar/vCard conventions</s:message>[LF]
I don’t know what a “Mimedir file” is, but it seems this is an OwnCloud error and not a DAVdroid error, especially when it’s returned on a REPORT addressbook-query request.
I have the same problem. This used to work fine at some point. I have a Nokia 7.1 with 8.1 Oreo. When I configure DAVdroid and then leave my phone alone for just a few minutes, synchronisation is disabled (both in the Android setttings and in DAVdroid). It seems that this problem is not exclusive to DAVdroid.
I think I may have found the solution while typing this message: There is a background activity manager in Android, you need to explicitly allow background activity for DAVdroid there (Settings->Battery->Background activity manager - sorry if the names are not correct, this is a reverse translation from my German Android UI to English).