Problem: Getting an error similar to "Unable to validate user" when trying to Save & Publish in the Hosted iFrame app.
Solution (Safari): If you use Safari as your primary browser, you will need to make sure you have 3rd-party cookies enabled. That can be done by going into the Preferences. Select the Privacy tab and click the Never option under "Block cookies:" (as shown below)
Alternate Solution: If you continue to get the error, please try to clear your browser cookies. The following article has tips on how to do so on multiple browsers: http://www.wikihow.com/Clear-Your-Browser's-Cookies