ssh.service: Unit entered failed state

Question

sshd starts, runs and works fine, but only once. If I try to log in on a second ssh terminal session I get rejected:

ssh: connect to host dmz port 22: Network is unreachable

And/or when I log out of ssh, the service dies:

Jun 25 10:47:16 dmz sshd[29221]: Received disconnect from 10.0.0.101 port         57874:11: disconnected by user
Jun 25 10:47:16 dmz sshd[29221]: Disconnected from 10.0.0.101 port 57874
Jun 25 10:47:16 dmz sshd[29221]: debug1: do_cleanup
Jun 25 10:47:16 dmz sshd[29221]: debug1: do_cleanup
Jun 25 10:47:16 dmz sshd[29221]: debug1: PAM: cleanup
Jun 25 10:47:16 dmz sshd[29221]: debug1: PAM: closing session
Jun 25 10:47:16 dmz sshd[29221]: debug1: PAM: deleting credentials
Jun 25 10:47:16 dmz sshd[29221]: debug1: audit_event: unhandled event 12
Jun 25 10:47:16 dmz systemd[1]: ssh.service: Main process exited, code=exited, status=255/n/a
Jun 25 10:47:16 dmz systemd[1]: ssh.service: Unit entered failed state.
Jun 25 10:47:16 dmz systemd[1]: ssh.service: Failed with result 'exit-code'.

Answer 1

Looking earlier I've just seen this:

Jun 25 10:46:44 dmz sshd[29221]: debug1: Server will not fork when running in debugging mode.

... which looks rather suspect.

Checking processes I see:

/etc/sbin/sshd -D -d

Checking /etc/default/ssh I see:

SSHD_OPTS="-d"

which I have commented out.

I do not know why it was set, I do not not remember ever setting it and ssh worked fine until I upgraded 14:04LTS to 16:04LTS.

ssh now works as expected.