I’m having the same problem since upgrading to 2.0:
“The originally uploaded image file cannot be found at ””
… for all images in my media gallery when I do a full force regenerate.
Also, I tried reverting to 1.5 and all thumbnails regenerate properly, so it’s definitely a recent change causing problems.
I’m also on Win7 x64, WAMP/IIS, WordPress 3.8.