Environment: Veeam 9.5 Update 4
Virtual Machines: Windows Server 2012 R2

Problem

Hopefully this will help others. I received the following error when performing a backup of two VM’s.  At this point nothing had changed and typically a retry or active full will resolve the issue. Not in this case

Processing <vm> Error: the specified item was already backed up or backup was not completed. Failed to download disk. Reconnectable protocol device was closed. Failed to upload disk. Agent failed to process method {DataTransfer.SyncDisk}

Error: the specified item was already backed up or backup was not completed. Failed to download disk. Reconnectable protocol device was closed. Failed to upload disk. Agent failed to process method {DataTransfer.SyncDisk}

Resolution

After trawling through similar issues i delved into the logs and found errors relating to the VMDK’s  I found a post (i will circle back to it when i find it) that said that if the VMDK’s have the same name the backup will fail

So after checking the two VM’s i did indeed have the same VMDK name. So i performed a Storage vMotion on the OS drive in this case which added _3 to the VMDK and voila a successful backup

If anyone has more details on this please share in the comments