It's not really a Grommunio related problem -- I know that. However, the official ISO should deny installation if the VM does not support EFI. On Proxmox for example, you will later run into problem -- as described in my first post. As mentioned above, I created a new VM and -- by accident -- and forgot to switch to EFI. Shit happens. 🙂
So yeah, just a suggestion to improve the official ISO and make it fool-proof.