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.