Problem

Your Linux Client was migrated to Debian Stretch and after the first login the screen remains black and nothing more happens.

Solution

It does not matter if you have a vanilla (empty) account or an account with an old KDE/Plasma profile in it.In Debian Stretch there is a bug which prevents the proper first start of Plasma. To circumvent this problem you have to stop the current session and then login again.

• Press CTRL-ALT-F1 to switch to the console.

• Login with your credentials

• use loginctl list-sessions to check the running sessions. If you have just login through Plasma and have opened a console you should see two sessions. The session under tty1 is the console one, the other one is the Plasma Session:

  $ loginctl list-sessions
     SESSION        UID USER             SEAT             TTY             
           3      16320 username         seat0                            
          49      16320 username         seat0            /dev/tty1       
  
  2 sessions listed.

• Now you can kill the hanging session (in the above example the first one with SESSION ID 3) with loginctl terminate-session 3

• After the Login Manager has restored, login again.