Also, in the sandbox test, the email requested at paypal checkout is the paypal buyer email address with -buyer appended to your paypal email address. You have to set password for this email in the profile of your sandbox account under where that email address appears. Your browser will  probably try to use your normal paypal email address for testing and that will not work. 

Hi William, thanks for the suggestion. Trimming those values will be added in next version. Regarding testing it is best to do that in private mode in browser. Best regards, Nikola