This is normally caused by some element on your page that's too wide. When you set element widths in px values, they don't scale. If you have an element that's 500px wide but the viewport is only 300px wide, you'll end up with a white gap on the right side of your site.
In some cases, it can be caused by an element that's animated in from the side using the Animate on Scroll feature. You can tell if this is the case, because the gap will go away once you've scrolled to a certain point on your page. If that seems to be the cause, you can add overflow: hidden to the container of the animated element to fix the issue.
If you're unable to identify the problem yourself, please e-mail firstname.lastname@example.org and we'll be glad to assist.