I recommend copying whatever you're posting/editing and exiting your browser, then going back in and trying again. This glitch happens to me a lot once I've been on the forum a long time without closing my browser, but exiting and going back in seems to fix it for me.
Other times, just copy what you're posting and go back to where you where before the error screen, refresh, and try again. Hopefully one of these will work for you, that error screen is pretty annoying.