FontAwesome icons don't load

I am hosting FontAwesome myself and it's working throughout the site. However, the icons don't get loaded in the editor, only a placeholder with the unicdoe is shown.

Has anybody else run into this issue yet?
<img width="57" alt="unicode in editor" src="https://user-images.githubusercontent.com/19467302/60397034-d1b8a380-9b48-11e9-98a4-2af14d72d0f2.png">
 