Hello!
It took me “forever” but I found something that was breaking Stripe’s Form (Card Number Field) but only for some users (I could only replicate the problem with a VPN but some users could not access the card number field even without one).
If you use the plugin All in One WP Security and you have “Deny bad query strings” turned on inside your Firewall > PHP rules > URL Security some users are unable to run s2member’s php that calls for that field, for some reason (but not all, hence why the mistery).
If you want your Stripe form to work properly turn that feature OFF (like in the screenshot above). Unless @clavaque finds a bug in the code that makes s2Member compatible with it again.
I wonder if that’s also the reason PayPal buttons sometimes don’t work.
I am glad I was able to finally find it after work on it on and off for months. 
I wish this helps someone.