ah! I've seen this before!
Check the emails / customer entries.
It has happened to me when there is an "invisible" character in a preceding message and this plays merry hell with the entries after.
The solution for me was to cut the content of (can't remember if last good message, or first bad one), into notepad to stip out the "weird stuff", and then paste the content back into the message.
Very non-tech response, but hopefully you get the idea.