Gap on right beyond page elements

I’m converting an only slightly modified Twenty-eleven site (https://ilahec.uic.edu/) to an unboxed version with images on wide color gradients and a mobile menu.

The new version looks ok on the staging site, but there is a horizontal scroll bar and a white gap on the right. I’m not seeing in my css or Inspector any elements calling for that. It is the same in all browsers, as well as with all plugins deactivated.

On the one page with a big image and animation (https://ilahec.uic.edu/Staging/ahec-scholars/) The gap is larger. If I set the #ahec-banner div to display:none, the gap shrinks but does not go away. That div is one of the elements I added to header.php, but even if I remove them all, the gap remains. (On desktop pages come up with the gap off-screen, and it only displays when you scroll right.)

I was going to post in Twenty-eleven but it looks like there hasn’t been a topic answered there in more than a year.

The page I need help with: [log in to see the link]