One further follow up: My guess is that this the blank screen issue may be occurring under selected circumstances only. Something like this:
1. We have seen this in IE - but it may not be the case with other browsers.
2. The browser has been left open while the underlying session has expired. This might be immediately after an upgrade or it could be later on.
3. The cookie contains information that is relevant to the old version 13 release - but not to the new one.
4. When refreshing from the URL, the old cookie values are sent to the server. The server rejects them with the error reported above.
5. At this stage, changing the value of the URL, hitting refresh or hitting shift refresh in the browser are all ineffective. Only the deletion of the the cookie plus a refresh will solve the problem.