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.

-------------UPDATE--------------------------------------------------

Link on select image button is the following:

index.php?option=com_media&view=images&tmpl=component&asset=com_content&author=&fieldid=jform_images_image_intro&folder=

I believe that the empty variable folder= is the reason for this issue, but I don't know how to fix this.