No longer able to insert links in post/page editor.
Since the WordPress 3.9 update I found the problem to be with "Ultimate Nofollow" plugin.
I tried deactivating/reactivating and the problem did not go away.
I tried deleting the plugin and reinstalling it and the problem still persists.
The only way to get things working is to completely delete the "Ultimate Nofollow" plugin.
Will you let us know when your plugin is fixed. Thank you.