Visual Editor not working anymore, only HTML editor works

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 (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?