Receiving errors when trying to copy files to VMFS utilizing Veeam Backup & Replication/FastSCP

The issues is a firewall issue most of the time try doing the following:

Veeam Backup and FastSCP uses SSH (TCP port 22) and/or HTTPS (TCP port 443) protocols as the control channels (from console to source and/or target) and the range of ports from 2500 to 5000 as transmission channels (between source and target). For one job (copy/backup/replication) one port from that range is used.

In case of using VSS, Veeam Backup uses NetBIOS (TCP port 139, UDP ports 137, 138), SMB (TCP port 445) and RPC (TCP port 135) protocol connections from console to guest VM additionally.

You need to open only a small range of ports for the concurrent jobs, depending on your environment. For example, you need to open 2500-2510 between source and target to be able to do 10 concurrent jobs.