Visual mode doesn’t work

Hi,
because I didn’t find a solution in the archive here…..

I use to write my posts with the Classic editor in classic mode. No block editor.
When I open the editor in visual mode, I cannot type or paste text in the text window. I have to move to text mode to add text. Then, going back to visual mode, the text window does not show anything.
See two screendumps here:
https://chaosowl.net/Classic1.jpg
https://chaosowl.net/Classic2.jpg
How can I get editing in the visual mode working correctly?

Regards,
Gunivortus

The page I need help with: [log in to see the link]