Thanks for your questions.
We solved the problem by applying the hint from nickvergessen in the following Question:
URL: NC12 Authentication issue with Cardav, Caldav and Sogo connector
Hint:
apps/dav/appinfo/v1/webdav.php
$authPlugin->addBackend($bearerAuthPlugin);
with:
// FIXME I disabled this myself following NC12 Authentication issue with Cardav, Caldav and Sogo connector $authPlugin->addBackend($bearerAuthPlugin);
apps/dav/lib/Server.php
$authPlugin->addBackend($bearerAuthBackend);
with:
// FIXME I disabled this myself following NC12 Authentication issue with Cardav, Caldav and Sogo connector $authPlugin->addBackend($bearerAuthBackend);