Same issue here.
I have read somewhere that it was in connection with use of shell fish.
I do the following test.

• Install fresh kubuntu 21.10 on virtual machine (vmware)


• do full updates


• install shell fish (at this stage all is working)


• change default shell for the user to fish


• disconnect from plasma and reconnect (i have the issue)


• switch to a console alt-ctl-F2 and change back the default shell to bash


• reboot the machine

After change back to bash plasma start without any issue.