Unfortunately the problem slightly came back again, but I asked another question to the forum in another thing, because I wanted to make the semi-transparent background of the top menu & app grid intransparent and 100% black - edit: for simple optical taste.
I got answer and was told by @Levone1 to edit a config file. Since I did so, also the sticking screen did appear never again!
See Thread Q: Can I make background of app grid and top menu full black? - Post #4
So maybe the sticking screen comes from a processor overload caused by the necessary graphics calculations for the screen effect? (that is now no more necessary - so no more overload?)
edit 15 days later: The app grid + the upper menu never again sticked since I changed the background color as written above.