Paypal Troubleshooting FAQ

I get a Message 3005 error from Paypal (“We are sorry, we are experiencing temporary difficulties…”)
The transaction works, but I get an "INVALID" message from Secure-eBook
The transaction works, but I get an "FAIL Error:4003" message from Secure-eBook
The transaction works, the email with the activation key is sent, but my "Thank You page" is not shown