#578 is caused by https://github.com/owncloud/contacts/issues/581 which is present since at least OwnCloud 6.0.3.
Please follow up in the issues mentioned above.
How often can DavDroid sync without severly impacting battery life? I ask because caldav on my iphone syncs very quickly (within 1 minute) but yet the default sync interval for caldav-sync is 6 hours.
Oh I think I see. It looks like with Android 7.1 the min sync interval is 15 min. Is this right?
Yes, that is the fastest interval that is allowed from Android - therefore its also the minimum that can be set in DAVdroid.