There are two programs so far that when added to the system cause that version of Mageia 6 to become unusable - smplayer and openshot. I mean that the process of adding them causes the problem, not running them.
The first indication of a problem is that System Settings is no longer available. The message is, "KDEinit could not launch /bin/systemsettings5." The second is that the system can only be shut down from the command line.
An attempt to reboot brings up the Mageia screen. There is a message I've never been able to read very well because it appears right at the top of the screen and disappears immediately: "[x?...] does not support boot process [? . . .]." Then a set of instructions comes up to inform me that there has been a problem starting my graphical display. If I follow the directions and enter "drakx11," the test shows everything OK. Getting out of that, there is the message "xauth: file/boot/.Xauthority does not exist" before I'm allowed to enter "systemctl default," which takes me back to the same set of instructions.
My computer is a Dell Precision Tower 3620. The internal graphics support is Intal HD Graphics 630, and the monitor is a Benq GL 2760. I have let the system set up the graphics, and that appeared to work just fine.
I have two versions of Mageia 6 on the computer - one is this system, meant to be my working system and the other is a nearly pristine system installed in the same manner. (I've given the second one access to a printer and chosen single click.) Each has been given mount points that allow access to the other. This arrangement gives me a second system that will function even if something bad happens to my normal one. That way I don't lose access to the Internet or the printer, and I can examine and perhaps repair the problem using all the facilities of the alternate system without resorting to the command line.
The pristine version is working just fine, and so was the one I'm trying to prepare for daily use until I tried to install one of the programs shown above. I haven't been able to find what needs to be fixed, and I've been poking around for a couple of days. I don't really need either of the programs that caused the problem, but I don't like the possibility of running into the same problem if I try to install some other program in the future unless I can find out what is wrong and how I can repair it without reinstalling.