Link back from PayPal works, but displays an error

Purchased Membership Pro today and you can see it at rpgkc.org/join

I've successfully configured everything and got it working without much difficulty. However, after payment is processed at PayPal, the user is not automatically taken back to my site. They must click a button to return to the site. When they do that, the site home page displays with an an error message: "Message: Invalid subscription code". This is the point that the site should get acceptance of the subscriber's payment, create a subscriber, sync with Joomla's user groups, and send notification emails and invoices. But none of that is happening.

The behavior is consistent across PayPal testing mode and live mode.

I searched through the forum and didn't find anything to help, so perhaps its something on my end. But I can't imagine what.

This turned up with a similar error message, but didn't seem to be the exact same problem:
joomdonation.com/forum/membership-pro/46334-weird-problem.html

Any help is appreciated.

Thanks,
Brad