Hi.
I have a problem on my KDE Neon 5.20 version.
The symptoms are:
1- The title bars have disappeared.
2- The windows are blocked, I cannot move them, or minimize or maximize them, or resize them, just close them.
3- The windows keep the order in which they have been opened, if I access one of the ones that have remained at a lower level, the upper ones are still on top.
4- In system configuration, there are different sections that launch error messages.
5- Others, such as menu windows that only appear for an instant, do not allow writing until the application is restarted, ...

I attach some images.



I have been doing tests with the Composer configuration and the different options of the appearance menus. I have also tried to find solutions on the internet, but I have not found them, probably because I have not been able to do the proper search.

BigNet has given me the solution here
https://forum.kde.org/viewtopic.php?f=66&t=90354#p432878