Migration fails: Cannot request remote agent status using command line

Applicable to:

The migration fails with one of the following errors:

PLESK_ERROR: Error message or request description : Error: launchpad error (Error code = 2): == STDERR ==================== Cannot request remote agent status using command line /var/cache/2013100110274185/PleskX.pl: Died at AgentConfig.pm line 23. at AgentConfig.pm line 23 AgentConfig::AUTOLOAD() called at /var/cache/2013100110274185/PleskX.pl line 298 main::writeMigrationResult() called at /var/cache/2013100110274185/PleskX.pl line 304

OR

PLESK_ERROR: Cannot request remote agent status using command line /var/cache/20150318093912860/PleskX.pl: which: no ruby in (/usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin)
Can't open perl script "/var/cache/20150318093912860/PleskX.pl": No such file or directory
Unable to start the migration with the following error:

PLESK_ERROR: SSH connection to the source server can not be used: there is unexpected output when executing commands. Clean output is required by migrator to execute commands and consume their output, and to run "rsync" tool (for file transfer) which uses SSH connection as transport and does not work if there is any unexpected output.

Specify the "root" credentials for the source server in order to start a migration. Alternatively, choose to authenticate via SSH keys. For details, refer to Authentication by SSH (Linux).
Log in to the source server via SSH.
Make sure that the PermitRootLogin option in the file /etc/ssh/sshd_config is set to "yes" or commented:

# grep PermitRootLogin /etc/ssh/sshd_config
#PermitRootLogin yes
Restart the SSH service:

# service sshd restart

Note: It is possible to migrate with a non-root user. For details, refer to Migrating From a Linux Server With Custom Hosting.