Migration failed: Command execution failed on the source server 'source' with non-zero exit code command: cat /tmp/version

Symptoms

Migration failed with the following error:

PLESK_ERROR:Failed to fetch basic information about resellers, clients and domains data from source servers
Cause: Command execution failed on the source server 'source' (203.0.113.2) with non-zero exit code.
command: cat /tmp/version
exit code: 1
stdout: ***hidden***
stderr: ***hidden***
That is a critical error, migration was stopped.

OR

PLESK_ERROR: Failed to fetch basic information about resellers, clients and domains data from source servers
Cause: Command execution failed on the local server with non-zero exit code.
command: rsync -r --chmod=Fu=r,Du=rwx,go= --timeout=30 -e 'ssh -i /usr/local/psa/var/modules/panel-migrator/sessions/20180209094058/ssh-keys/id_rsa.192.0.2.2 -p 22 -o PasswordAuthentication=no -o StrictHostKeyChecking=no -o GSSAPIAuthentication=no' /usr/local/psa/admin/plib/modules/panel-migrator/backend/lib/python/parallels/plesk/source/legacy/extras/plesk_17_0_pmm_shared/ user@192.0.2.2:/home/user/plesk_migrator/plesk_migrator-8mgjmlp7briynooadwq4gqktq71mhpyb/pmm_agent
exit code: 255
stdout:
stderr: Permission denied (publickey,password).
rsync: connection unexpectedly closed (0 bytes received so far) [sender]
rsync error: unexplained error (code 255) at io.c(600) [sender=3.0.6]
That is a critical error, migration was stopped.

Migration was started from a non-root user on the source server.

Unable to open /tmp/version file on the source server:

CONFIG_TEXT: Permission denied

Cause

Not enough permissions for running migration under a non-root user.

Resolution

Start the migration from a root user.

See the Migration Guide for more details.