Exchange 2010 OWA. 2 load balanced Client Access Servers.
Error message: Your network connection is unavailable. Please reconnect and try again.
Another indication of the problem was that the none of the buttons in OWA work.
So…slightly embarrased, but this was caused by putting OWA in to a url redirect loop. When I set the default website to redirect to https://webmail/owa the change propogates to all of the subsites, including /OWA. So the /OWA site had a redirect to https:///webmail/owa.