I also had the same error:
WARN - Virtual machine <MYVIRTUALSERVER> is currently migrating and will not be backed up
I tried multiple things, rebooting vCenter, re-registering the plugin, reinstall VSC and recreating the job. They all failed. I even tried migrating the VM to another host and datastore. Still didnt' work. Finally I removed the VM from inventory and reregistered it in vCenter. The backup worked again.