I'm following this issue on Launchpad (Bug #477169). So far, the most succinct diagnosis of the problem:
The problem is now identified, grub2 in some cases recognizes loop0 as an ext2 when is an ext4 (and so it is unable to read it properly)! But is strange how it is taken so much time to find a solution...
However, I feel that the workarounds presented for this issue are beyond the ken of the average user. I gave up trying to fix it using those methods because they were just too time-consuming. So I suggest two things that have worked for me so far:
1) Back up your work files.
2) For Wubi installations, do not run any upgrades that might touch Grub2 or initrd (typically these will involve kernel updates.)