According to the logs:
EXCEPTION java.io.IOException: stream was reset: REFUSED_STREAM
this is a duplicate of nginx HTTP/2 incompatible with okhttp. Please follow up there.
I’m using the actual davdroid version and nextcloud 11.0.1.
I have an suscribed ICAL calendar in nextcloud. It called “Abfallkalender”.
But on my android smartphone this calendar is not synced.