Whoops, looks like you skipped a step! Please go back and try again

I have installed latest version of Wordpress and marketpress. I have set up the store. I keep on getting: "Whoops, looks like you skipped a step! Please go back and try again." error when I fill in Paypal info or if I log in paypal to make the payment. I have tried the manual payment. There is no other plugin active. I have changed the session timeout to be 10000. I have read other threads and tried all the solutions. This issue is holding me back from the launch. Please advice.