Admin theme broken after update to 5.9.3?

Hi @ll!

I’m helping a fried with her site and it seems that the admin theme is broken since the automatic update to 5.9.3. The site itself (content, theme 2016-child, editor) seems working fine though.

The admin pages on the other hand is quite broken: menu entries missing, buttons empty, drop down lists only partly with named entries. The update page is mostly empty after I updated one listed plugin.
Site language is German but most is in English now. I logged in with English and two submenues of the plugins of “Calculated Fields Form” and “Master Slider” are still German.
The confirmation text if something is successfully done or there is an error in the menu has no text, only a green or red marking left hand side.

I already disabled all plugins (blindly and with browser dev tools) but nothing changed. “WP Super Cache” is installed but inactive since I noticed some side effects.
The “Health Check & Troubleshooting” plugins sais everything is fine (except of the disabled plugins and time zone setting).
The “Enable jQuery Migrate Helper” did not seem to help either.

Hosted at Strato with PHP 7.4.28.

Does this ring any bell? What should I try?
I can add some screenshots if needed.

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