I love this theme (and Fictive), but I am experiencing the same problem with both of them. At particular viewport widths, a white strip appears down the right-hand side of the screen. This will be off-screen initially, but a horizontal scroll-bar appears so that this area can be revealed (be scrolling to the left).
I have tested this on my MacBook, where the glitch appears when using Safari but not Firefox. I have also tested this on an iPad2, an iPad Mini and an iPhone, all of which use Safari and all have the same problem. It's especially bad on the iOS devices, as the strip becomes visible when the screen is pinch-zoomed.
I've tried deactivating plug-ins, to no avail.
My website is: jameswhitman.co.uk
Hope you can help.