Thanks for the update.
Hmm... I'm sorry it's taking me a while to figure this one out. But I'm not giving up.
Have you tested using a custom return URL in the shortcode? I remember seeing mention of someone having success with that attribute to avoid the error you're getting, for some reason.
I am guessing you already did all the common troubleshooting steps? https://s2member.com/kb-article/common-troubleshooting-tips/
One thing I'd try at this point, just out of the fact that only a few users are reporting this problem out of thousands of installations, is to install in a separate server if you have it. I know the server scanner now clears, but maybe there's a factor it's not catching, I don't know...
So, if you have another host to test with, could you try installing a clean WP there with just s2Member, configure it with PayPal, create the same button, and test a purchase?