CKEditor doesn't work in multiple browsers

I am using the OpenCart version from GitHub ( https://github.com/opencart/opencart ) and in the admin panel I cannot get the text to show up. The problem is cross-browser, albeit inconsistent:

• IE11 (currently relevant): nothing happens
• Chrome 37.0.2062.124 m (currently relevant): My options (bold, italic and underline) to toggle on every key press

Any hints, similar ideas, etc.?

Edit:

GitHub bug link: https://github.com/opencart/opencart/issues/1934