You SSH in, the connection is accepted, then instantly closed — no password prompt, no key challenge. A frequent culprit is an AllowUsers (or AllowGroups/DenyUsers) directive in sshd_config that no longer lists the accounts you actually log in as.
Read the auth log
# on the server (console/out-of-band access): tail /var/log/auth.log # or journalctl -u ssh # "User admin not allowed because not listed in AllowUsers"
Fix it without a second lockout
Edit /etc/ssh/sshd_config so the directive lists the right accounts, validate the config, and reload rather than restart so existing sessions survive if you got something wrong:
AllowUsers admin deploy sshd -t # validate config syntax first systemctl reload sshd # apply; keep a second session open as a safety net
Always keep one SSH session open while changing sshd_config — if the change locks you out, you can revert from the session you still have. For a replaced-host-key error instead, that's a different problem (client-side known_hosts).
Practise it
SSH lockouts are one of the simulator's access incidents. Work them in a full incident response shift.