I can answer my own question. Sorry for asking, but I was already pondering several hours …
the package qt5ct is included in the openbox-metapackage. This is not compatible with kde !!!
When uninstalling openbox, I thougt that qt5ct was part of kde anyway, so I didn’t uninstall it.
But: kde-openbox cannot be used together with “normal” plasma!
I wanted to install openbox to test something. Plasma is my main desktop environment.
After logging out of openbox I saw that there is a kde-openbox session too. So I tried that. Big mistake.
kde-openbox totally messed up my regular plasma-desktop. The two main problems:
- after logging out and in again, always a Numix-Icon-Theme is used, no matter what I select in system settings
- in dolphin I have to use double-click, no matter what I set up in system-settings
So before using a backup (I did quite some work after the last backup) I would like to know if someone here knows how to revert the changes that were made by the kde-openbox session.
By the way, this definitely is a bug. On opensuse, which I used before, kde and kde-openbox could be used on the same system for the same user.
The strangest thing: I thought that this was some config-problem in my home-dir. So I added a new user, and even though openbox was uninstalled again, the problems were the same with the new user.
Thanks for your help,