I have had that problem for quite some time. When I go to reply to a topic or start a neww topic all I get is the page all the same brown color. It will not open page for message and load the stuff.
What I do is click to post a reply or for a new topic. Wait for it to load or not and see at bottom of my screen for done message. If it did not open message box when it says done I will hit refresh page while the page is still there for messages. That generally does the trick. If not I get pissed and move on.

Try "allowing pop up's for this site" at the top of your browser. This might be the problem.