(They say it's a bad sign when you start talking to yourself. Does responding to your own posting in a forum count?)
It appears to be an issue of zip timeout caused by assignment submission size. I was able to reproduce the problem by loading a bunch of large submissions to my test assignment. The blank screen mentioned above showed up at exactly 60 seconds.
This Tracker issue is probably applicable: https://tracker.moodle.org/browse/MDL-40245