I also read the the mentioned article of William Lam and I edited the vmx-file manually ...I did not use SSH...I edited the file with an editor and changed the value. I'm not that clever using SSH or Powershell...so only to see if it works I tried it editing my way (using an editor)....and finally the error message "ACPI motherboard layout requires EFI. Failed to start the virtual machine" was gone after doing this....so obviously it worked !?
Do you think the third error message ("Operating System Not Found") is related to the fact, that I edited the vmx-file in a different way ?
Update: I just managed successfully to recover a Win-10-PC in vSphere Client from the same Active Backup for Business-Backup (the Active Backup for Business-Backup contains several Win-10-machines, several Win11-Machines and one Windows Server 2019-Machine).
So obviously the issue is linked to the win-11-pc. First thing which looks different -->it was noticeable that, in contrast to the Win11 restore, two entries (here the PC "Anmeldung") were generated in the inventory list

This wasn't the case with the Win 11 PC (here the second entry was missing ("without parent element"),
Otherwise the VM started immediately - i.e. in UEFI mode and without having to make any changes to the VM's settings