in my case the popbox container has always a width of zero. After a quick lookup I found that you use $(settings['box']).width() to determine the width. But if you have more than one popbox (therefore more than one .box) on your site its better to use $(settings['box'], this).width(). For me it fixed the problem.
I don't want to open a pull request for that litte think, but if you want i'll to that :-)