I know Kubuntu (20.04) is probably not supported but it works - until you install Parallels tools. After that the VM hangs at boot. Lucky for me I learned long ago to 1) Snapshot my test systems and 2) tread carefully with new Parallels and Linux. OTOH - VMware Fusion (and Workstation) seem fine. Probably issue with the virtual graphics driver Parallels uses.
Have nearly the same problem. After reboot there is only a black screen. Pressing Alt+F2 switches to a console where i can login. Entering "startx" on the command line fires up plasma and all works fine. Any Idea to get this going automatically? BTW default target is graphical.target Help would be very much appreciated.
Change to boot in console mode by $ sudo systemctl set-default multi-user.target After the login, startx by .bashrc.