Thanks for writing back.
I tried what you suggested. I opened two tabs in my browser and went on to the Pro editor in one tab. I changed the CSS code on that one page and saved it. Then I purged the W3 total caches from the other tab and also the Cloudflare cache. Unfortunately, that didn't resolve the issue. The CSS code still comes back when I reload the Pro editor.
What do you mean by "do not forget that you install Rocket script"? Are you talking about Cloudflare's Rocket Loader set to automatic if that's what you are talking about. But I also have a page rule in Cloudflare for tv-magden.ch/x/ with two settings: Cache Level: Bypass and Disable Performance. This should always disable the Rocket Loader for Pro's editors too, right?
Any other ideas?
Minify is not activated, not in W3 total cache nor in Cloudflare. So, the issue for the other error must be something else. But what?
Thanks for further assisting me.