Plesk Migration error: Failed to check SSH connection to the source server 'source'

Symptoms

• A Plesk migration (Tools & Settings > Migration & Transfer Manager) is currently stuck at "Check connections" phase.

• The migration info.log file on the target server shows the following error:

  # less /opt/psa/var/modules/panel-migrator/sessions/20230105213105/info.log
  [2023-01-05 21:31:07][ERROR] Unable to execute command because another migration is in progress.
  [2023-01-06 15:01:37][INFO] START: Check connections
  [2023-01-06 15:03:47][ERROR] Unable to connect to '203.0.113.2' by SSH: [Errno 110] Connection timed out. Retrying in 10 seconds
  [2023-01-06 15:06:07][ERROR] Unable to connect to '203.0.113.2' by SSH: [Errno 110] Connection timed out. Retrying in 10 seconds
  [2023-01-06 15:08:26][ERROR] Unable to connect to '203.0.113.2' by SSH: [Errno 110] Connection timed out. Retrying in 10 seconds
  [2023-01-06 15:10:45][ERROR] Unable to connect to '203.0.113.2' by SSH: [Errno 110] Connection timed out. Retrying in 10 seconds
  [2023-01-06 15:13:05][ERROR] Failed to check SSH connection to the source server 'source' (203.0.113.2): Unable to connect to '203.0.113.2' by SSH: [Errno 110] Connection timed out.
  Ensure that the server is up and there are no firewall rules that may block SSH connections to the server, then restart migration.

• An SSH manual connection from the target to the does not work:

  # root@plesk-server:~# ssh -v root@203.0.113.2 -p22
  OpenSSH_8.2p1 Ubuntu-4ubuntu0.5, OpenSSL 1.1.1f 31 Mar 2020
  debug1: Reading configuration data /etc/ssh/ssh_config
  debug1: /etc/ssh/ssh_config line 19: include /etc/ssh/ssh_config.d/*.conf matched no files
  debug1: /etc/ssh/ssh_config line 21: Applying options for *
  debug1: Connecting to 203.0.113.2 [203.0.113.2] port 22.

• Telnet from the target also does not work:

  # root@plesk-server:~# telnet 203.0.113.2 22
  Trying 203.0.113.2...
  telnet: Unable to connect to remote host: Connection timed out

• Target IP address is not blocked by Fail2ban or is in the allowlist (whitelist).

• Target IP is not blocked on source firewall, and SSH connections are allowed.

Cause

Network connection issue between servers

Resolution

Contact the network administrator