According to https://groups.google.com/a/opencast.org/d/msg/users/yM37S9MpHO8/pINQeL_dBQAJ a required change to provide sakai user integration (https://opencast.jira.com/browse/MH-10806) was missing on (https://bitbucket.org/opencast-community/matterhorn/pull-requests/133)
As discussed, this Issue is intended to solve that.