Seems to be an exception in Android code, caused by syncing tasks without permission (i.e. when Tasks is installed after DAVdroid).
Davdroid fails since Android 7.1
I upgraded to Android 7.1 today and since then DavDroid fails to synchronize with NextCloud. Here’s my debug log: https://git.project-insanity.org/snippets/10
This is not related to updating Android. Please follow up at https://forums.bitfire.at/topic/1259/rev-must-not-appear-more-than-once-in-a-vcard-component/