Sorry, scratch my previous post. It does not show the problem with the copy. Although there is an error with the root device (which actually happens on both VMs) the VMs seem to boot fine.
However, now I think I can reliably reproduce the real problem. I think the problem appears when I create a snapshot while the VM is running:
$ lxc launch ubuntu:focal ubuntu1 --vm --target host1
Creating ubuntu1
Starting ubuntu1
$ lxc snapshot ubuntu1 test
$ lxc stop ubuntu1
$ lxc cp ubuntu1 ubuntu2 --target host2
$ lxc start ubuntu2 && lxc console ubuntu2
To detach from the console, press: <ctrl>+a q
BdsDxe: loading Boot0001 "UEFI QEMU QEMU HARDDISK " from PciRoot(0x0)/Pci(0x1,0x1)/Pci(0x0,0x0)/Scsi(0x0,0x1)
BdsDxe: starting Boot0001 "UEFI QEMU QEMU HARDDISK " from PciRoot(0x0)/Pci(0x1,0x1)/Pci(0x0,0x0)/Scsi(0x0,0x1)
error: unknown filesystem.
error: no such device: /.disk/info.
error: no such device: /.disk/mini-info.
GNU GRUB version 2.04
Minimal BASH-like line editing is supported. For the first word, TAB
lists possible command completions. Anywhere else TAB lists possible
device or file completions.
grub>
Note that I do not copy the snapshot but the whole VM after it was stopped. So I am not sure why a snapshot can cause a problem.