Hi Uwe,
This error happens when a 3rd party script is causing a fatal error, and blocks further script rendering on the page. It is extremely hard to debug without actually seeing the issue. Usually de-activating plugins one-by-one is the way to go, but sometimes custom scripts might be the cause.
On your homepage I see a fatal script error, it can be the cause, if that script is also loaded on the back-end: https://i.imgur.com/QsAcWGx.png
..further expanding that error reveals this: https://i.imgur.com/U1Rgxtq.png
The cause there is a text, where the apostrophe in the word “don’t” is not escaped properly: “Sorry, this doesn’t work on Periodics®.”
Removing the apostrophe from that text might help, if the issue is indeed caused by this error. It is possible that there might be other scripts loaded on the back-end.
