Thank you for all your replies.
@gruen from what you wrote, it is much easier to wipe the endeavour os system partition and start fresh again.
@rabcor Not sure if the “ubuntu and windows used the shared EFI partition” was true. The efi partition was created automatically on the same drive as the Ubuntu. Besides Ubuntu related partitions, there was just one more partition on that drive that i made in order to save there downloaded and installed games through Steam on Windows. So technically there is no reason to windows put EFI parition (if it even has to create one) there. In fact, i was running Windows just fine, then i bought this 1TB drive where i made the “gaming” parititon and rest of unallocated space i gave to newly installed Ubuntu. Back then both, Ubuntu and Windows were booting just fine. It has to be Ubuntu what created the EFI partition and populated it with data. That means there must exist a way to remake the same parititon with same contents and functionality by using Endeavour OS. I don’t think proceeding a repair would help anything, in fact, it even could harm existing fully functional system by editing those system files. I CAN boot windows, just not within grub. Instead, i need to boot the drive where is the windows system located / saved.
EDIT:
I checked the grub entry again and in the name it says “Windows 10 (on /dev/sda1)”. I installed GParted and checked what partition it is refering to. It is refering to parition on my main drive for Windows user data - a separated drive for user data like program files, downloads etc. This drive has 2TB in size and majority of it is allocated to one partition for mentioned user data. It has second, small partition, reserved by system (probably Windows particulary) of size 500 MiB. Thats all. The /dev/sda1 partition is this particular system-reserved partition with attribute boot and size 500 MiB as i mentioned eariler. So it doesn’t make sense it would want to boot into Windows by booting into that partition.
I checked the UUID of the parition as well and it matches the partition i mentioned here. The system-reserved one on drive for windows user data. No sense to boot it.
EDIT 2:
To confirm my theory, i entered boot menu and entered the drive matching /dev/sda. Voila, it showed the Windows error screen i shared here some messages above. This confirms booting into that drive doesn’t make any sense and instead, grub should address the right partition.
Now the question changes a bit. How can i say to the grub configuration generator that i want to boot windows not from automatically discovered partition but from manually set partition instead?
