I managed to finally solve this by reinstalling Windows and then installing Ubuntu 18.04.3 again. I don't know what the difference was when I installed Windows myself(as opposed to the preinstalled version) but doing this certainly solved the issue. I was able to dual boot after that.
** Changed in: grub-installer (Ubuntu) Status: New => Invalid -- You received this bug notification because you are a member of Ubuntu Bugs, which is subscribed to Ubuntu. https://bugs.launchpad.net/bugs/1839894 Title: Installing Ubuntu 18.04.3 LTS alongside Windows 10 (Dual Boot) causes grub install to fail with fatal error To manage notifications about this bug go to: https://bugs.launchpad.net/ubuntu/+source/grub-installer/+bug/1839894/+subscriptions -- ubuntu-bugs mailing list ubuntu-bugs@lists.ubuntu.com https://lists.ubuntu.com/mailman/listinfo/ubuntu-bugs