Hi.
I guess this is an s2member issue.
The reason “Checking Credentials…” is hanging like that is most likely because s2member is trying to redirect, and the Modal login plugin is waiting for a response, which it isnt getting. s2member will only respond to the redirect_to var or disable the s2member login redirection totally, so I suggest you try that, to get any popup-login-plugins to work.
Try do disable s2member redirecting;
<?php
add_filter(‘ws_plugin__s2member_login_redirect’, ‘__return_false’);
?>
put this in your (child)theme’s functions.php file.