Error: VDDK error: 21036749815809.Unknown error
Failed to upload disk.
Agent failed to process method {DataTransfer.SyncDisk}.
Exception from server: VDDK error: 21036749815809.Unknown error
Unable to retrieve next block transmission command. Number of already processed blocks: [172772].
Failed to download disk.
This error looks to be coming from inside of VMware vStorage API... do you have this error every time you do a backup? Have you tried switching to different backup modes (network, hotadd, SAN)?
I had the same error on one of our VM's after the upgrade to v8, I just ran it on network mode and it completed fine, I wanted to see if it was fixed in patch 1 before I logged a ticket. it only happened on one VM out of 500 so I was not too worried. I will let you know if patch 1 fixes it.
Same issue here on our Exchange server. All other jobs are running fine. Have reset CBT multiple times. Using SAN direct mode. Coming from EMC NetWorker using exact same servers and it backs Exchange up just fine.