GRUB confuses kernels/initramfs of Ubuntu and Arch Linux installation

I finally solved it by nuking the Arch partition and its boot files in my Ubuntu's /boot directory from orbit. Ubuntu is fine again now, all remaining GRUB entries are working again.

Here's a list of what I did:

• Delete Arch's initramfs files:

  sudo rm /boot/initramfs-linux*
  

• Delete Arch's vmlinuz file:

  sudo rm vmlinuz-linux
  

• Format the Arch partition (/dev/sda8) using GParted

• Update GRUB's configuration:

  sudo update-grub
  

• Reboot and enjoy!