I've been using Add to Menu without an issue for years. This afternoon, when I click on the Add to Menu button, a pop-up window appears but it is blank. I've tried on multiple browsers and on multiple websites. They are all on Joomla 3.6.5. I tried re-installing it and I also upgraded to the Pro version. Neither of those actions helped. Any ideas?
(I also re-booted my computer, just in case you're wondering.)