I have several plugins installed, some free, some purchased. All of a sudden I was no longer able to add them to my pages. When I click to add e.g. YouTube PRO the browser page becomes dark. I can see the options dialog, but when I click, the browser darkness disappears and so does the dialog.

I have tried on Safari and Firefox on two different Macs running El Capitan, cleared cache and updated the WP installation (4.7.4). All plugins are up to date.

Steps to reproduce:

Login to WP admin page.
Click on pages
Click on a page
Click on one of the plugin buttons
Page goes dark (a lightbox opens?) and the plugin settings dialog is visible
click anywhere
Lightbox disappears and you are back to the Edit page page.