hmmm!
I'm having the same issue ! !
Have built many Windows VM on Unraid in the past, but have spent the last 2 hrs trying to install Windows 10 21H1, as I couldn't update from Win 1909 but it goes to Shell everytime.
This is the my first Windows install since Unraid v6.9 and have tried re-downloand the windows iso changing the different VM configs, eg OVMF vs SeaBios, IDE vs SCSI, but I cannot get it to work
I can create Linux VM's without issue, just Windows that isn't working