I don't see what I could have done wrong. All I was trying to do was to split my one large file (and it worked yesterday when I did it). I put my cursor where I wanted my next chapter to begin (I didn't delete any text), and clicked the Chapter break button.
The error message says,
The operation you requested can not be performed because Section0001.xhtml is not a well-formed XML document.
An error was found on line 1363, column 5, expected end of tag 'body'.