The test that I'm suggesting you do is not to create a new message and see if it is sent but to see if you can add anything to the body of the message using the edit with DW button. If you can add in some simple text as a test and get through using the server behavior then it may indicate an issue with the form that is being used. If the result is the same please let me know and I can open a ticket for you so that this issue can get resolved.