1. Edit this post:
2. switch to "code" (in the editor)
3. look for this code:
4. remove the p tag at the end and insert a div tag. The result should look like this:
It is indeed an unclosed div tag just before the more tag.
The "more" tag ist that function which allows you to break a post with a "(more...)" link. If you add it to an existing text it might produce such errors. I use to insert it on a new line at the end of a paragraph, then it does not break anything.
Edit: Probably you won't be able to see the "embed" code: look for the audio tag you inserted.