Comment # 2 on bug 1124027 from Stefan Hundhammer

So, that library appears to be needed directly (unlikely) or indirectly (more
likely) by a part of YaST. 

I suspect that this might have to do with one of the desktop plug-ins that the
Qt libs use for better desktop integration: In Qt, a desktop can provide its
own file dialogs with such a plug-in. Installing another desktop like you did
might have caused that: Now it might be looking for the GNOME file dialogs
plug-in which may no longer be present or which might have been overwritten by
those provided by the KDE Plasma 5 desktop.

Please check if one of those commands complains about any missing shared
library:

YaST Qt UI:

  ldd /usr/lib64/yui/libyui-qt.so.9.0.0

  ldd /usr/lib64/yui/libyui*

Qt libs:

  ldd /usr/lib64/libQt5{Core,Gui,Widgets,X11Extras}.so

If there are complaints, first try "sudo ldconfig" to rebuild the linker cache.
That might (!) already fix it.

If this does not help, please paste the output of the command that fails here
(and please don't forget to reset the "needinfo" flag when you are done).