Paypal redirect error

I have had multiple prospective recurring payment users on my s2member Pro enabled site reach out to say that when they signup via a paypal button on the membership options page, and complete the purchase it redirects them to an error page. They then can not register. The paypal purchase itself goes through without issue. I then have to look up their transaction and manually create a membership registration access link for them to complete their profile.

A membership options page, auto-return url and Paypal PDT token have been set up correctly. The customer does not receive an email via S2member after purchase, only the standard paypal email. I can’t figure out the issue!

Running a cache plugin, but when I remove that- issue remains. I have IP Blacklist plugin installed, but that is all for security. Also, was advised to try async javascript plugin. I have a paypal business account.

This has only started happening within the last 2 months. It worked fine before that. Any help / advice?

Many users of s2Member have this problem since last September, the solution would be to go through a PHP syntax instead of the shortcode.
You can read an example of a topic on this problem here: Click Here!