I found a problem when upgrading to ESXi 5.1 that Veeam won't back the servers up giving a "Error: Host with uuid '12345678-1a23-1b23-1234-123456789101' was not found" message. Having found http://tsmith.co/2012/veeam-job-fails-after-upgrade-to-vsphere-5-1/ (thanks Tim) he tells you how to sort it in an older version, but I couldn't figure out where to fix it in Veeam 6.5.

It's in File > Help > License > Licensed Hosts, you need to revoke on each host then run your backup jobs as normal, it will re-add them back to the list. Hope this helps!