I'm not entirely sure in your case, but I have seen this error before when I have accidently left an extra blank line above my HTML when copying & pasting the code.
Interesting on the blank spaces.
The error message specifically refers to Editable Areas that allow you to use the Editor properly. If the DIV tag is wrong, it could break something when the Editor is used.
Probably most likely to occur as a mistake or with the Blank Template method when you drop in code and don't care that much about Editable sections.