Just recently I had some time to work on the problem again.
I have chosen the first option, as it seems to be the most simple. I have downloaded boot-repair-disk (I could not find the download button for Remix), and I followed the procedure of repair. I have got the info:
"GPT detected. Please crate a BIOS-Boot partition (>1 MB, unformatted filesystem, bios_grub flag). This can be performed via tools such as Gparted. Then try again"
The true is that I have installed sparky using a manual partitioning with GPT partition table.
I have checked bios of my laptop, and it seems that it is a standard bios, but UEFI compatible. So I have reinstalled the sparky using Bios partition table. I also left 2 MiB of empty unallocated space, before the first partition. The installation went without problems, as I could recognize. The system did not boot from the pendrive. The pendrive has not even been recognized as bootable.
So, the problem is not solved.
Additionally, I have found that the kernel works on only one core of the processor (whereas it has 4 cores). It is, therefore, the next problem to solve.
Unfortunately, I do not have so much time to spent on it. So I decided to give up with the sparky for now.
Thanks to all for help.