Just want to add that I'm experiencing same issue randomly.
I have seen it on 2 totally different systems, in Backup and also in Replication jobs.
Details about one instance of the problem:
Error: Client error: End of file Unable to retrieve next block transmission command. Number of already processed blocks: .
After retry it works, but every several days one different VM might have this or similar problem (I don't always have time to investigate further).
VBR server = physical Dell R610 (acting as the proxy and repository).
VBR version = 126.96.36.199
Production storage = HP 3PAR 7200 with 4 FC connections.
Production environment = 3 hosts, VMware esxi 5.1 update 1.
Backup job in SAN mode (using FC card on backup server), using reverse incremental.
Backup Repository = IBM DS3300 iscsi storage, mounted directly by VBR server.
I have also received report from another client of us, he had the same error but with replication job to remote site.
He has an HP P4300G2 (lefthand) ISCSI SAN on production site.
So I assume the problem is related to reading data from source storage.
Maybe the problem is related to SAN mode?