It seems like I'm encountering the same issue as you. Trying to backup a VM running SBS2011 on ESXi, through Veeam. I'm also getting "Unable to retrieve next block transmission command", and I'm getting it at the exact same block every single time. My other VM's are backed up without issues, but these aren't very big in physical size. My SBS2011 is around 500 gigs, so it takes a while to reach the error (almost at the end of the job, every time).
I also have a support ticket running, and so far the supporter asked me to try and up the memory for the VM from 4 to 8 gigs. I just did that, and I'll see how it works out tonight when the job runs again - unless the support comes up with something before that.
I'll try and keep you posted on this. I'm thinking, that if I get a resolve on my problem, it might help you as well (and vice versa)