PayPal Button: success parameter not working?

I’ve just upgraded to s2Member Pro to be able to create a custom Thank You page with my standard PayPal button. However, when adding the success="" parameter to the PayPal button shortcode, after finalizing the purchase, neither my customized thank you page nor the standard thank you page with register account link are shown. Instead another thank you page is shown, telling the user to check his email for further instructions.

This is the PayPal Button shortcode:
[s2Member-PayPal-Button level="1" ccaps="kurs0001" desc="Member / Description and pricing details here." ps="paypal" lc="" cc="USD" dg="0" ns="1" custom="" ta="0" tp="0" tt="D" ra="0.01" rp="1" rt="M" rr="1" rrt="" rra="1" success="" image="default" output="button" /]

This is the non-customized thank-you page we get:

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