We had a situation in which someone created a Google Form and the preview for it would always come up with a resource unavailable error.
This wouldn't happen for any other form. And it wouldn't happen on another computer.
Clearing the cache didn't fix it, but clearing just Google cookies did fix it.
Something definitely worth trying, since most of the results for this indicate a problem with the Google Docs service being down and nothing to do with cookies being corrupted.