Login with LightDM fails

Question

I try to login with LightDM on Kubuntu 16.04. LightDM is the only installed display manager. I see the login screen and enter my password, but I get the message Failed to start session.

The last view lines of /var/log/lightdm/lightdm.log are:

[+0.88s] DEBUG: Session pid=1134: Greeter connected version=1.18.1 resettable=false
[+1.05s] DEBUG: Session pid=1134: Greeter start authentication for fabian
[+1.05s] DEBUG: Session pid=1188: Started with service 'lightdm', username 'fabian'
[+1.05s] DEBUG: Session pid=1188: Got 1 message(s) from PAM
[+1.05s] DEBUG: Session pid=1134: Prompt greeter with 1 message(s)
[+1.20s] DEBUG: User /org/freedesktop/Accounts/User111 added
[+9.33s] DEBUG: Session pid=1134: Continue authentication
[+9.69s] DEBUG: Session pid=1188: Authentication complete with return value 0: Success
[+9.69s] DEBUG: Session pid=1134: Authenticate result for user fabian: Success
[+9.69s] DEBUG: Session pid=1134: User fabian authorized
[+9.69s] DEBUG: Session pid=1134: Greeter requests session kde-plasma-kf5
[+9.70s] DEBUG: Seat seat0: Failed to find session configuration kde-plasma-kf5
[+9.70s] DEBUG: Seat seat0: Can't find session 'kde-plasma-kf5'
[+9.70s] DEBUG: Session pid=1134: Greeter start authentication for fabian
[+9.70s] DEBUG: Session pid=1821: Started with service 'lightdm', username 'fabian'
[+9.70s] DEBUG: Session pid=1188: Exited with return value 0
[+9.70s] DEBUG: Seat seat0: Session stopped
[+9.70s] DEBUG: Session pid=1821: Got 1 message(s) from PAM
[+9.70s] DEBUG: Session pid=1134: Prompt greeter with 1 message(s)
[+13.18s] DEBUG: Seat seat0 changes active session to 
[+20.54s] DEBUG: Seat seat0 changes active session to 1
[+231.22s] DEBUG: Seat seat0 changes active session to c1
[+231.22s] DEBUG: Session c1 is already active
[+282.08s] DEBUG: Seat seat0 changes active session to 1

Previously, I used sddm to log in which worked fine. I'm aware of lightdm failed to start session; user folder requesting Gnome but moving the users .Xauthority file didn't change anything.

I tried to purge and reinstall lightdm but that did not help.

I also tried these suggestions without success: https://askubuntu.com/a/223634/458355

~/.xsession-errors does not report errors.

What can I do to make login with LightDM work?

Solution

Inspired by https://askubuntu.com/a/504402/458355 I created /etc/lightdm/lightdm.conf with the following content:

[SeatDefaults]
user-session=plasma-kde

Now login works.

For the record: The file ~/.dmrc containes the line

Session=kde-plasma-kf5

However, changing this line to Session=plasma was ignored by lightdm.