It’s hard to see the exact values and setup of your constraints as you’ve pasted them here, so I’m not sure from looking at your screenshots where you have gone wrong.
In lieu of an explanation of what’s wrong in your setup, I’ve created a basic sample project with a very similar view hierarchy and constraint setup to the one you describe. The horizontal scrolling works as expected in the sample project, which uses the “Pure AutoLayout” approach that Apple describes in the Technical Note.
I also had a lot of trouble originally getting Auto Layout to work with UIScrollView. The key to getting it to work is making sure that all of the items in the scroll view, taken together, have constraints that eventually link to all sides of the scroll view and that contribute to the AutoLayout system being able to determine a contentSize for the scroll view that will be bigger than its frame. It looks like you were trying to do that in your code, but maybe you had some superfluous constraints in there that were making the contentSize too small.
Also of note, as others mentioned, with AutoLayout and UIScrollview, you no longer set the contentSize explicitly. The AutoLayout System calculates the contentSize based on your constraints.
I also found this ebook chapter to be very helpful in making me understand how all this works. Hope all this helps.