I've reproduced same issue on VM. It seems that I have go perform `full-upgrade` that removes `user-manager`. After that KDE desktop works.
If I try to install `user-manager` again, I get suggestion to remove whole kde-plasma-dekstop & friends...
- Bug#978008: libkwaylandserver5: undefined symbo... Vincas Dargis
- Bug#978008: libkwaylandserver5: undefined ... Vincas Dargis
- Bug#978008: libkwaylandserver5: undefined ... Norbert Preining
- Bug#978008: libkwaylandserver5: undefined ... Vincas Dargis
- Bug#978008: libkwaylandserver5: undefi... Norbert Preining
- Processed: Re: Bug#978008: libkway... Debian Bug Tracking System
- Bug#978008: marked as done (libkwaylandser... Debian Bug Tracking System
- Bug#978008: marked as done (missing breaks... Debian Bug Tracking System
