TinyMCE does not display the content properly

Steps to reproduce:

1. Go to Hippo Acceptance Project channel
2. Click on the xmas tree document from the menu
3. Edit the xmas tree document
4. See the introduction field (compare it to how CKEditor displays it). See screenshot

Expected result:

TinyMCE displays the HTML content properly, and not as String. This means that all the paragraphs are formatted correctly, images are shown in the editor, ...

Actual result:

The HTML source code is displayed. See screenshot