This problem has occured at my site lately too, It seems like it's those who want to pay using their own paypal account who excpreience this problem. Everything is configured right, and my primary e-mail adress is correct, but still they get this error message:

"We cannot process this transaction because there is a problem with the
PayPal email address supplied by the seller. Please contact the seller to
resolve the problem. When you have the correct email address, payment can be
made at www.paypal.com."

This is very frustrating because I have new customers everyday. It's so strange because I haven't done any changes since I started.