Marketpress Paypal IPN failure & blank screen at checkout

Hello! I have a client that I've installed marketpress on. I had some issues early on, but seemed to have worked out the kinks... until now.

When I do a purchase at http://mqumag.com/backissues/ I can get through all the screens fine. My client however, cannot. I'm on a mac, she is pc, dont' know if that information is vital or not.

Anyway, she gets a blank screen once she gets to the payment screen. I see things just fine and cannot duplicate her error.

Also, I got this message from paypal:
Please check your server that handles PayPal Instant Payment Notifications
(IPN). IPNs sent to the following URL(s) are failing:

http://www.mqumag.com/shop/checkout/paypal/ipn/

If you do not recognize this URL, you may be using a service provider that
is using IPN on your behalf. Please contact your service provider with the
above information. If this problem continues, IPNs may be disabled for your
account.

Thank you for your prompt attention to this issue.

Thanks,
PayPal

So I temporarily disabled the IPN notifications until I could figure this out. Any advice?