Preview does not display changes made in editor

Steps to reproduce:

1. create a new Blog post.
2. in the edit view type in some content
3. choose the "Preview" button

This displays an error message instead of the content:

Similar:

1. go back to edit, and click "Save & Continue"
2. now the blog entry exists; make changes to the content and the title
3. choose the "Preview" button

The change to the title is displayed, but the change to the content is not.