Keep getting error message when trying to send a private message, please help

I'm trying to send a private message to one of your members, but after I compose the message, I get an error message saying:

Correct the highlighted errors and try again.

  • Your post has been changed because invalid HTML was found in the message body. The invalid HTML has been removed. Please review the message and send the message when you are satisfied.

There are no highlighted errors. No matter what I change in the text, I get this message. 

What should I do? Thanks