Most common problems could be:
Modifications in SSH config files /etc/ssh/sshd_config
How to debug
On the server:
– You could run another SSH process on different port, and monitor console log.
$ sudo /usr/sbin/sshd -d -p 4444
– Connect to the newly instantiated SSH process on port 4444 (-p 4444) with -v (verbose) option and monitor the log.
ssh -v -p 4444 firstname.lastname@example.org -i ~/.ssh/tmux_ssh
Once you find the issue and fix it, you could restart the SSH server with the following commands:
$ sudo launchctl unload /System/Library/LaunchDaemons/ssh.plist
$ sudo launchctl load /System/Library/LaunchDaemons/ssh.plist