After pressing the power button, the Grub Boot menu appears.
If you press return, the error sometimes appears:
error: command failed
error: you need to load the kernel first.
The hard disk contains three partitions.
An EFi, an empty one for Windows and
one for Linux Mint.
If the notebook is switched on after a long period of time,
it can boot right away without any errors.
After a reboot you always get to the Grub command line.
I'm not sure whether the problem comes from Grub2 or whether
the EFI settings do not match. The notebook was originally
delivered with Ubuntu 18.04. So there were no problems with booting.