I found the solution!
It is not a flaw of Davdroid, it is because of the new dav-URL. The old url which needed to be used for caldav was https://hostname.com/remote.php/caldav
But with Owncloud 9.0 and newer it is now for caldav and carddav https://hostname.com/remote.php/dav
So the solution is, delete your old account in davdroid and recreate it with the new url. Details here
If you use the wellknown-notification, keep in mind to change the owncloud-apache config too, like this:
Redirect 301 /.well-known/carddav /owncloud/remote.php/dav
Redirect 301 /.well-known/caldav /owncloud/remote.php/dav