When trying to start Veeam Backup Free Edition, you get this error:
Start-VBRZip : Veeam Backup Free Edition does not allow VeeamZIP of multiple VMs. Specify a single VM, or install a paid product license
This is because Veeam Backup is already running against another VM on the host, or another copy of Veeam Backup is running on the backup server (check task manager and close other open versions).