I understand this is an old thread, but if someone reads this, it may help. I had been digging into this issue for a few hours now and here's what I have found:
This is strictly referring the original scenario with Server 2012 host
Referencing Veeam article: http://www.veeam.com/kb1855
Veeam requires Hyper-V Integration Services version 6.2.9200.20873 for guests 2008 and above. Even with all Windows updates installed, the highest I am getting is 6.3.9600.16384
I found this Microsoft hotfix that updates the Integration Services version on the host: http://support.microsoft.com/kb/2878635
After installing the hotfix on the host, try updating Integration Services again on the guest VM. With a few reboots in between of course, try the backup again.
Hope this helps someone out there!