It's only on the private messaging function. I've had it happen before, it may ask you to log in again, and then take you back to the compose a message page, albeit with an empty text box. My recommendation is to use ctrl+c to copy the text you typed before sending and then ctrl+v if it sends you back to the compose a message page (not sure what the commands are in a non-windows environment, but every operating system should have copy and paste shortcuts).
Alex-- it's an authentication error of some sort, I haven't seen it in weeks, but if the issue hasn't been solved the next time I see it I'll post a screen shot here. My initial thought when I saw it was that the PM functions use a different user authentication method than the forum itself and thus a different user session, but that's just a guess.