library not visible and/or accesible in class notebook

I am a teacher in the Netherlands. Since august 2019, I'm having problems with accessing the library (wich contains the learning materials) in my class notebooks. The items on the left side (= library, collaboration space, student names) don't appear when opening the class notebook (online in browser). This happens on different machines (win 8 pro, win10 enterprise) on different locations (home, school) and in different browsers (firefox, explorer, chrome).   Refreshing /restarting the browser doesn't help. It's a matter of chance wether I can see the library or not. And once I see it, I can't open it (in different pc's win-versions, browers). Sometimes it can take to 20 minutes before I get lucky and can open the library again.

In this video I made  (located in my  onedrive)  you can see what goes wrong: https://tinyurl.com/y3r26n8w

The problem only occured with me, but since today, my students complain that they have the same problem: they can't access the learning materials either.  Since the whole curriculum is in class notebooks, this is a major problem. Can anyone help?  Thanks!  Richard