Cannot complete checkout; page times out

If you place an item in your cart, then go to the checkout page, then click "checkout now," it never takes you to the actual checkout page. It goes to this URL: https://www.rrus.org/store/shopping-cart/shipping/ and I get this message:

"The page isn't redirecting properly

Firefox has detected that the server is redirecting the request for this address in a way that will never complete.

This problem can sometimes be caused by disabling or refusing to accept cookies."

It doesn't have anything to do with cookies, I don't think. it's happening in multiple browsers many times.

Any ideas what could be causing this?