Having Trouble Getting the Payment Gateway to Work


I'm having problems getting PayPal Standard Gateway to work. As far as I know, I have it setup properly. I have my merchant ID in there, have it activated, and followed the IPN setup instructions.

When I hit subscribe, it takes me to PayPal, where it asks me to login. Once I login to complete the order, it says "The link you have used to enter the PayPal system is invalid"

I've tried both sandbox & live site modes, same results. Any ideas?