Could have something with the information you add into your signature and how it's listed. In all honesty I don't know quite for certain why or when this pops up, but some people get this from time to time. Could have something with how the tables list on the server your signature information.
Have you tried just changing your signature to something simple like plain text "Just some text" or similar and try to submit that? And if so, does it say the same thing or does it work then.