Verify there is not a blank space after the token in the setup fields:
Somehow copy and paste inserts a blank and it will not match what PayPal is expecting and then the PayPal box will close immediately.
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.