Since the update to WordPress 3.9, Magic Fields 2 causes a warning when publishing a new post. When I click the Publish button, my browser (tested with Chrome and Firefox) askes me if I want to leave this page. When you click 'I want to stay at this page', nothing happens. When you click 'I want to leave this page' you don't actually leave the page; the post just gets published as you expect it to do.
I disabled all my plugins to test which one causes the problem. It appeared to be Magic Fields 2. I downgraded WordPress to 3.8.x and the problem was also solved.