Blank page, 414 error, request URI too long
If you’re getting a blank page when redirected to the Membership Options Page, when returning from the payment gateway, or some other long URL, you may be getting a 414 error: Request URI too long. http://www.checkupdown.com/status/E414.html
What you need to do is contact your web host so they raise the limit for your website/account.
s2Member has a few URLs that can be quite long, and you need these to work or you’ll have trouble with some checkouts or redirections.