Hi guys, having the same issue here. Im having the latest version of WP, no conflict with plugins as I already tested it, latest s2meber framevork installed (free version), server configuration tested as suggested and all green checkmarks...
The only issue I may think of is that you requre https for IPN in s2member plugin and PayPal IPN still accept http. In my case, website is http and I set up http link in PayPal IPN. Everything works correctly beside this redirection message.
S2member sends confirmation email with registration link, it registers user, gives privileges, cancel status if subscription is canceled etc. So everything works correctly except return message after purchase.
My solution was to change default-return.php file in s2member/src/includes/templates/returns and put continue tag instead of response.
I know that it is not the best solution but at least I got rid of that annyoing message and users are redirected to Homepage instead of presenting error. Beside that, everything else works fine. I wrote explanation to my users to check emails after purchase and from there on everything works as it should.
Waiting for the fix and hope it will come soon. It is obviously conflict between PP and s2member.
I tried fix with paypal-return-in.inc.php and it didnt work.
I cannot use "success" tag with "thankyoupageurl" inside button because it would require to change more than hundred buttons which already exist on the website, so this is not solution for me.
I think one of the solutions would be to provide option to customize this return message for people who are getting this issue. Everything works perfectly, just this error message interupts the flow.