Ben Armstrong has posted an article explaining why a virtual machine might go into a “Paused-Critical” state. Quickly, it’s because Hyper-V is protecting a VM with a dynamic expanding or a differencing VHD (virtual hard disk) runs out of space to expand on the physical storage. Importantly, Ben also says:
“Due to a bug in the current version of Hyper-V – it will file warning messages and place the virtual machine into paused-critical even if the virtual machine is using fixed size virtual hard disks, and the drive that is storing the disk runs out of space for some other reason”.