We had a similar problem, only in reverse. Our ArcGIS server had no security certificate, but SSL enabled in AGO didn't allow for untrusted sources. Assuming you don't care about SSL (and if you're doing mash-up you probably don't ) you can fix by navigating from AGO user home to Edit Settings -> Security, Unchecking "Allow access to the organization through SSL only", and finally mashing the "Save" button at the bottom. If it doesn't prompt you, you might wanna log out and log back in for good measure.
Thanks for providing the clue re: security!