For some strange reason I cannot insert intro/full article image in articles anymore. 

 1. On "Images and Links" tab, I click on button to add images.
 2. Popup screen appears and then I paste image url in "Paste image URL in textarea, you can press Enter to add a line, one line per image" textarea.
 3. I click the Insert button and nothing happens

In Firebug Console I noticed that clicking the Insert button gives a javascript error:

**"ReferenceError: insertFormField is not defined"**

There's no reference to a js file that triggers the error. Any ideas?

PS. my joomla installation is up to date (version 3.6) and there were no new extensions installed.