The problem is caused on pages being served securely over https because editor-style.css has:
This means that the ‘secure’ page is trying to load insecure content. All you need to do is change it to https: (an unsecured page won’t care if secured content is loaded) or remove the “http:” entirely in which case it will load the url using which ever protocol the site is using. I.e.:
I have changed it in my own installation, but would appreciate it if you could fix it in the next update so that I don’t have to keep changing it or try to make a child to deal with the issue.