I'm not sure if this is what happened, but if you had opened her oto recently and accidentally clicked on some box saying "Shift-JIS" in the upper right corner... it apparently corrupts the heck out if you accidentally click "UTF-8" and save it. It's not fun, to say the least.
As for how to fix that, if you still have the .zip file for UTAU, try unzipping it again. It shouldn't be corrupted, especially if your locale is in Japanese and Defoko wasn't in mojibake before. If that doesn't work, there's probably other versions of Defoko floating around the Internet (there's an alternative oto
here, too).