This has happened twice now. Once yesterday, I tried to send a message and got the error. Clicking the "Back" button on my browser did get me back and the next attempt worked. Just now, I had sent a message and clicked on "Home" to check posts - same error. Same solution - click "Back" and try again.
It's not much more than a minor nuisance, but I wanted to point it so so you can make sure everything is running as it should.
(Btw - I'm stuck with I.E.6 on this machine, so it COULD be on my end.)