There are four situations that commonly cause this error:
The Veeam VM is on a different host than the source VM, and the VM was added to the job through the host directly (as opposed to through the VC).
Storage hosting disks of the source VM is not accessible by the ESX on which the Veeam VM is running
The Veeam VM and the source VM are managed by different VC's.
The Veeam VM and the source VM are managed by the same VC, but located in different DataCenters
In the first case above, add the affected VM to the job by the VC.
In the other situations, however, Hotadd/Virtual Appliance mode is not applicable or possible given the current VMWare architecture.
What's going on essentially, is that the host (or cluster) is unaware of the other VM to be able to find it. Hosts and Clusters both are isolated elements that aren't able to talk without a higher point - VC. Otherwise, when a disk is requested for attachment, it cannot find either source or destination (depending), and fails with a "UUID is not found/Parameter is not valid" type error.