I’m having the same issue. See http://prntscr.com/10cjki
The box pops up but doesn’t show any widgets. This had been working until we migrated the site and domain to a new server. I’ve already tried reinstalling the plugin as well as the others. And no new plugins have been added that it wasn’t already working fine with.
Unfortunately because of security issues I’m not able to give out an Admin account temporary or otherwise. Can you give me a clue as to where to look to fix the issue. What was the issue with the person listed above?