Found the solution:
Entering "set" in the grub console shows the prefix points to "EFI/ubuntu" instead of "EFI/kubuntu". Boot the system, copy the folder "/boot/efi/EFI/kubuntu/" to "/boot/efi/EFI/ubuntu/" and everything works fine.
This is definitly an error in the default installation configuration - should be fixed as fast as possible!
I have no idea where the grub configuration is placed.
Found the solution: efi/EFI/ kubuntu/ " to "/boot/ efi/EFI/ ubuntu/ " and everything works fine.
Entering "set" in the grub console shows the prefix points to "EFI/ubuntu" instead of "EFI/kubuntu". Boot the system, copy the folder "/boot/
This is definitly an error in the default installation configuration - should be fixed as fast as possible!
I have no idea where the grub configuration is placed.