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 and there were no new extensions installed.