There is a long reported issue from 2015 about not being able to use the same Trial Amount and Regular Amount with Pro Coupons, due to PayPal and Authorize.Net-imposed limitations.
But the same logic is still present in stripe
Attempting to set the ta="" , tp="" , and tt="" Trial Period attributes to the exact same value as your ra="" , rp="" , and rt="" Regular / Recurring attributes will result in the following error in stripe as well, as can be seen on lines 491 and 492 of s2member-pro/src/includes/classes/gateways/stripe/stripe-responses.inc.php
else if($attr['ra'] && $attr['ta'] === $attr['ra'] && $attr['tp'] === $attr['rp'] && $attr['tt'] === $attr['rt'])
$response = array('response' => _x('Invalid form configuration. Invalid "ta, tp, tt" attributes. Trial Period. When provided, these cannot be exactly the same as your "ra, rp, rt" attributes.', 's2member-admin', 's2member'), 'error' => TRUE);
This is despite a KB article saying stripe does not have these issues
https://s2member.com/kb-article/why-cant-i-use-the-same-trial-amount-and-regular-amount-with-paypal/
Could @clavaque confirm if these lines in s2member-pro/src/includes/classes/gateways/stripe/stripe-responses.inc.php are supposed to be there or not.