@SaWin, I had the following output from boot-repair. Will reboot now and see what happens. (And maybe edit this answer.)
Edit 1: YAY! GRUB showed up and I'm posting this from the installed Ubuntu. Now let's see if booting Windows (8.1) doesn't re-f### EFI.
Edit 2: If I select "Windows Boot Manager (sda2)" I am still able to boot Ubuntu later. I will not try the other options GRUB came up with.
An error occurred during the repair.
Please write on a paper the following URL:
http://paste.ubuntu.com/p/VjSBmy9Xmm/
In case you still experience boot problem, indicate this URL to:
boot.repair@gmail.com
You can now reboot your computer.
Please do not forget to make your BIOS boot on sda2/EFI/ubuntu/shimx64.efi file!
If your computer reboots directly into Windows, try to change the boot order in your BIOS.
If your BIOS does not allow to change the boot order, change the default boot entry of the Windows bootloader.
For example you can boot into Windows, then type the following command in an admin command prompt:
bcdedit /set {bootmgr} path \EFI\ubuntu\shimx64.efi