Symptoms

Unable to start migration. Downloaded debug log shows following error:

2016-02-05 02:32:16][ERROR] Failed to fetch basic information about resellers, clients and domains data from source servers
Cause: The command "
                        authorized_keys_dir="`dirname ~/.ssh/authorized_keys`";
                        if [ ! -d "$authorized_keys_dir" ]; then
                                mkdir -p "$authorized_keys_dir";
                        fi;
...........
executed at the source server 'source' (xxx.xxx.xxx.xxx) returned a non-zero exit code.
Stderr:
bash: line 8: /root/.ssh/authorized_keys: Permission denied

Immutable bit is set for /root/.ssh .

# lsattr -a /root/.ssh
----i-------- /root/.ssh/.

Cause

Plesk is not able to create authorized_keys because i attribute specified for /root/.ssh .

Resolution

Remove i attribute:

chattr -i /root/.ssh