I was having some trouble on Ubuntu 20.04 with figuring out why a Active Directory user couldn't log in to the server via SSH, and eventually came up with this, which was inspired by the link below it.
sudo journalctl | grep <<username>>
From here:
https://serverfault.com/questions/130482/how-to-check-sshd-log
All messages about sshd:
journalctl -t sshd
or
journalctl -u ssh
Messages about sshd from the last boot:
journalctl -t sshd -b0
Messages about sshd from the last boot in the reverse order:
journalctl -t sshd -b0 -r
