It seems like the page keeps getting redirected to itself, until you get the error.
That page seems to be the one used as the Membership Options Page, which should always be public, but at the same time is protected at Level 0 (logged in user), so the not-logged-in person gets sent to the Membership Options Page, which is the same, and you get the loop. WP Admin > s2Member > General Options > Membership Options Page
Please check in your configuration what restriction is protecting that page at Level 0. That should not happen, and s2 actually prevents it, as far as I know, but there seems to be something doing it. WP Admin > s2Member > Restriction Options
Please tell me what you find.