It's interesting that the seamframework.org site suffers from the same problem.
If you have any page open for too long, then try to login using the controls at the top of the page, you just get an error message.
The only workaround that comes to my mind is to make the login a GET request.
This issue does affect the user experience. Would like to hear from other folks and the Seam Dev. Team.
I assume that what you're seeing is a ViewExpiredException, which is standard behaviour for JSF. You should be able to work around this, by overriding the restoreView() method in the view handler and creating the view if it doesn't exist. I found a good example of doing this on someone's blog here.