Problem with PayPal communication and IPN Status says Retrying with HTTP Response Code 404

Hi, I have a membership website with membership plugin using the latest version. I am using the PayPal Single Payments Gateway. I have IPN enabled on my PayPal account and copied and pasted the URL on the Payment Gateway page into my PayPal IPN field.

I made 2 test transactions yesterday, and when I go to my PayPal account and to Profile > History > IPN History, I see both listed in the IPN History. Also the email account on my PayPal account did receive the email notification of the payment.

The status for both says "Retrying"

The two columns "Message ID" and "Transaction ID" are both hyperlinked.

When I click on Message ID for each, I see the following:
- The Notification URL is the same as what was listed on the Payment Gateway setting in the Membership plugin. It's https://mydomain.org/paymentreturn/paypalsolo
- The HTTP Response Code is 404.
- The Delivery Status is "Retrying"
- The No. of Retries is 12.
- The IPN Type is "Transaction Made"
- And there is a lot of information in the IPN Message, but I don't think I should paste on a public forum. Do you need to know?

When I click on Transaction ID for each, I see the following:
- The Payment Status says Completed.
- The Payment Type says Instant.

Can you help? None of my users can get to the special pages they can get to when purchasing a membership. Does it matter that my entire site is "HTTPS"?

Thank you.