WordPress “Page not found error” when I edit a page

What form are your pretty permalinks taking, and what is the title of that page? I’ve had something similar happen and it had to do with the page title. Try flushing your permalinks (go to Settings > Permalinks and switch to a different default, save, then switch back again to what you were using).

Though of course this doesn’t explain at all why putting the word ‘from’ in the body of the post would make any difference at all. 🙁