How do I permanently fix a GRUB partition off by one error?
Solution 1:
Grub is probably dropping to the console because the partition layout has changed since grub was first installed. Now that you have updated grub.cfg, re-installing grub should fix it. Boot into your ubuntu system the same way you did in order to run update-grub. From there, do a grub-install:
grub-install /dev/sdX
(Replacing X with your device, in your case it looks like it is probably /dev/sda.)