When trying to access the Business Data Services Application Proxy, I got the error "The server was unable to process the request...". I also noticed that the Secure Store service was giving a WCF-related error.

 At any rate, what I did to fix my particular error is start the "Claims to Window Token Service" which stopped for some reason. BCS started working again after doing so.

 This post is what pointed me in the right direction and gives more information:
http://www.intheknow.it/Default.aspx?Page=ssserror&NS=&AspxAutoDetectCookieSupport=1