I was trying to backup a VM using a Unitrends appliance. I kept getting the following:

WARNING: Did not receive any data!!

I found the following in the vSphere client:

Create virtual machine snapshot

The operation is not allowed in the current state.

So, it turns out the the issue was not with the Unitrends job, but the snapshot process on the VMware host.

I used ssh to login into the host as root and did the following:
/etc/init.d/hostd restart:

[root@host1:~] /etc/init.d/hostd restart
watchdog-hostd: Terminating watchdog process with PID 572484
hostd stopped.
Ramdisk ‘hostd’ with estimated size of 1053MB already exists
hostd started.

/etc/init.d/vpxa restart:

[root@host01:~] /etc/init.d/vpxa restart
watchdog-vpxa: Terminating watchdog process with PID 573196
vpxa stopped.

Then, I rebooted the VM and tried the backup again. It worked fine.