nicolas wrote: ... I suppose that it is just a web browser caching issue and if you clean the cache on your browser, you won't see the problem either.
This is so strange and not completely to explain. You are right with the clean demo installation you've logged in. The funny thing, I checked it before with different browsers and also cleared all Caches I can think of. System and Browsers. And the issue was still there.
1 day later, it was not visible anymore. At least in this particular installation.
I really can't see a rule here. Especially because it showed up in different browsers.

I also tried it on other installations, and the issue is still there.
Currently, I can't spend more time on it. And I don't see a 100% solution. I have to try it later again.
Until then..., thank you very much for your support and time!