I did just registered here minutes ago, just to say: Thank you! This solved my Issue as well. Unraid 6.12.10.
I just renamed my VM on the Unraid-GUI while the VM was down, and after booting the VM again, the VM suddenly couldn't find the vDisk anymore. The Unraid-GUI mysteriously loses the Function to change the Disk-Type once it's created.
Changing the XML-file also solved my problem: Somehow, the XML-File stated vDisk-Type "raw" all of a sudden. During Creation-Time, I am 100% sure that I changed that to "qcow2". It seems that just renaming the VM caused Unraid to change the Type back to "raw"...