I see. So you have nothing in the /wp-content/mu-plugins/ folder?
Are you using an s2Member pro-form or button with the [hilite mono]success[/hilite] attribute pointing to a URL?
[hilite path]Dashboard -› s2Member® -› PayPal® Pro Forms -› Custom Return URLs Upon Success[/hilite]
[hilite path]Dashboard -› s2Member® -› PayPal® Buttons -› Shortcode Attributes -> success[/hilite]
Did you enter a redirection URL for the Login Welcome page? [hilite path]Dashboard -› s2Member® -› General Options -› Login Welcome Page[/hilite]
If it’s none of the above, then it is possible that some other plugin added that redirection. This is not something s2Member does by itself, it’s either user input or another plugin. Please test deactivating other plugins one by one and checking after each if you still have that redirection. It’s better to use two browsers, one to make the changes and another to test, so you don’t log in and out of the admin account all the time.
I hope that helps!