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Could be a UEFI bug in the VM itself;
Could also be that you didn't sign your boot image since that command seems to load the secure boot signing key into the UEFI firmware, if you cleared other signing keys then potentially no code can load. You would have to load the keys for whatever UEFI firmware vendor is used (presumably that made by the VM software maker) or sign it yourself, etc.
I'd have thought that would happen the first time I enrolled the keys though since I used the exact same command both times?
Could be that you loaded an incomplete set the second time...? 🤷
When you reset "secure boot settings" did you clear the TPM contents? Would that have included a. private key used in the disc encryption? Then when you regenerated keys it will have been with a different seed and so different.
I don't know much about his stuff, but that bit sounded odd to me.
I didn't regenerate the keys I just re-enrolled them. I assumed the old ones were still in the file system since they were still being used to sign the UKI?
That would make it stop at the end of the bootloader with decryption failure, not full bricking
Is the underlying file system you're hosting the VM on encrypted as well? If so, that might be your problem.
What does sudo qm terminal get you?
No my host is unencrypted. Unfortunately I deleted the VM and I can't replicate this issue so I can't run that command. I obviously did something different the first time but I can't think what!
Likely it was tpm related. Good luck.
so I enabled the boot menu in the VM settings, entered it and reset the secure boot settings.
In BIOS/UEFI? What settings does this affect/what changes does this make?
Yes, on 
I unchecked attempt secure boot and reset the secure boot keys. This just disabled secure boot and the VM worked how it did before I set up secure boot.