ssh.service: Unit entered failed state

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'.