Cliff notes - having problems backing up a Hyper-V cluster and you've moved VMs around? Do a re-scan on the cluster IP in backup infrastructure.
I have recently migrated from VMware to Hyper-V and used Double Take Move. It's unfortunate that Microsoft has just released a converter that can do online conversions, but it wasn't available when I started my migration.
I want to share some experience with my migration. Double Take Move has worked well so far, but when it migrates a VM it doesn't put the configuration files and VHDs in the exact spot I would like. It doesn't add the virtual machine to the cluster either. When a VM is migrated and I have the integration tools installed and everything is running OK I will move the VM to the correct spot on my clustered shared volumes and add the VM role to the cluster to make it highly available. During the time when it is only on the Hyper-V host there is a good chance Veeam will back it up, especially if you have your backup server pointed to the cluster IP and not individuals hosts. This is fine and worked ok, but after you move it around and/or add it to the cluster Veeam gets confused. Long story short out of 18 VMs I had 9 warnings, 3 failures, and 3 successes. Yesterday I went into my Veeam backup server, backup infrastructure and did a "re-scan" on the host. All successes!!!! Doing this will update Veeam on the hosts, their volumes, and what's connected to it.
Any migration is scary, but stick with it, it'll work out...