The BIOS was earlier set to work both with legacy and UEFI, priorize legacy and CSM enabled. Debian was installed on the second disk and the debian-themed grub menu worked flawlessly - unfortunately I don’t remember how or where it was installed.
When installing Manjaro I did not have the choice for uefi or legacy.
Depending on the setting in the BIOS the laptop will now boot directly to Manjaro (UEFI mode) or to a grub menu missing the Win10 option (legacy/uefi with CSM). I’ll look into the option legacy only.
Choosing the boot drive directly did not result any change, I could not boot Win10 since the first time the grub rescue prompt came.
Perhaps not the best idea, but I started a new topic on the dual boot issue, when I saw the data being intact here. In that thread I included the output of some of the usually required commands - care to take a look and move this conversation?