You get "Please try closing and reopening your browser window." when posting a message on Facebook. But closing the window does not get this message to go away.
In my situation, this was due to having the Cross Origin Resource Sharing mode enabled on my Chrome using the CORS extension. By turning it off, the problem went away.
This is often considered as a non-secure mode and Facebook especially will not allow some features to be used like Posting and Messaging when the web request has been modified.
The CORS extension is used by developers like myself to access resources from different web servers and it is not normally needed for consumer use, so if you do get into this situation, you may actually have some other mechanisms/extensions which
Also try with a different browser and if it only happens on Chrome (or vise versa), then I will clean up the browser.