This problem can have many causes, so specific answers don't work too well. Try this for troubleshooting:

1. wait for the "A stop job is running for Session c2 ..." message to appear on shutdown and reboot after shutdown has completed
2. run journalctl -p5 in a terminal and press END to get to the end of the systemd journal (-p5 filters out a lot of garbage)
3. start a search by pressing / and enter the search term Stopping timed out. Killing.
4. search backwards by pressing SHIFT+N repeatedly
5. the line below each match tells you which application was misbehaving: Killing process 1234 (jack_thru) with signal SIGKILL.

If it is always the same application, you want to find out what it does and why it isn't stopped at shutdown. Otherwise it could be more complicated to solve the problem, but you might still get a hint or two.

Good luck! :)

This problem can have many causes, so specific answers don't work too well. Try this for troubleshooting:

1. wait for the "A stop job is running for Session c2 ..." message to appear on shutdown and reboot after shutdown has completed
2. run journalctl -p5 in a terminal and press END to get to the end of the systemd journal (-p5 filters out a lot of garbage)
3. start a search by pressing / and enter the search term timed out. Killing.
4. search backwards by pressing SHIFT+N repeatedly
5. the line below each match tells you which application was misbehaving: Killing process 1234 (jack_thru) with signal SIGKILL.

If it is always the same application, you want to find out what it does and why it isn't stopped at shutdown. Otherwise it could be more complicated to solve the problem, but you might still get a hint or two.

Good luck! :)