Visual Editor not working anymore, only HTML editor works

Hi,
this is not related to a WPMUDEV product, so its o.k. if you can't or won't help. But it never hurts to ask.
This site http://www.essenbildet.de (WPMUDEV dash installed and support access granted) has the issue that when I edit a post, the visual editor does not load. It just shows a blank page. HTML Editor works and also the Site Origin Page Builder works.
Any idea how to get the good old trusty visual editor back again?
thx
boris