I have multiple times a day where this happens and usually I just get tired of trying and stop replying to people. Which is a bit of a loss for the forums because I bet it happens to more than just me. (Not saying I am a loss to the forums, but I know there are others who provide a lot of help here that probably get turned off by the constant errors...)
At this point I always just copy my reply before hitting post. That way I can just refresh if it fails and paste it back in. But if it fails again I give up at that point...
Funny I got it again while typing submitting this post the first try...