Getting an error when placing order with Marketpress

I just received an email from a customer saying that they attempted to order a product and received an error after entering their information. I checked it myself and I do receive an error “There was a problem connecting to PayPal to setup your purchase. Please try again.

10002 – Security error – Security header is not valid”

I followed your instructions to the T in regards to connecting PayPal to my store. Please help; who knows how many sales I lost before this one person decided to take the time to email me about it.