Smart Reboot failed because ... VMs have their Suspend operation blocked. Would you like to force?

At first I thought this is because some VMs had enabled "Protect from accidental shutdown". So I disabled this for all VMs. However, even after a forced smart reboot, and after restarting all VMs I still do get the same message when I initiate a Smart Reboot.

What could be the root cause of this and how can I fix it?

I think is issue is what keeps me from doing a "Rolling Pool Update".