I’m trying to set up the Supporter plugin but with no luck. When I try to make a purchase in the PayPal sandbox everything works fine (it shows the correct subscription information on PayPal, etc) until I click the "Log in" button. On the next page I just get an error:
The link you have used to enter the PayPal system is invalid. Please review the link and try again.
And a button to "Return to Merchant" — when I click that button it takes me to the correct Supporter page with a "Transaction Canceled!" message.
– Checking all the "Supporter" settings, and they’re all filled in
– Creating a different sandbox account
– Installing a fresh copy of WPMU and supporter