No, i did that before..... I had thought that something was broken ... so i reinstalled Libreelec again from the scratch ... just to verify that videos can't be played at all ...
At least so i thought. Then i installed 3sat mediathek .... same result .....
And i installed ARD Mediathek ...
-- i have to correct my first post: it wasn't 'ARD-Mediathek' but 'Das Erste Mediathek which crashed -- .... and now i finally i got it working....
So defect addons '3sat-Mediathek' and 'das Erste Mediathek' (they must have something in common)
working addon 'ARD Mediathek'
Thank you, CvH