I'm fed up with this. I write a detailed message and press Submit, and I get that. I press back, and my composition is gone.
I think it happens most often when I've logged in only recently, i.e. I've read a few posts, decide to reply to one and log in. At that point, if I close everything and start again and find the thread I want to reply to then it works. But if I don't do that then I run the risk of being told I'm "unauthorized" and losing whatever I've written.
This isn't rocket science! Why do we put up with it?