I think there is something about the fact that the windows 7 was on a non uefi computer before

It's probably related to the fact that you're booting Windows 7 winload.efi with Windows 10 boot manager. Usually it works - newer boot managers recognize signatures of older Windows versions. I don't know why it's not the case here. You could try to copy winload.efi from Windows 10 to Windows 7, but make backup of the old one - transplants don't always go as planned. Yes, old computer booting with BIOS may have something to do with this.

I tried to delete winload.efi but its protected by trusted installer