BuddyPress.org

FYI, you currently can’t override /_inc/ajax.php in your child theme as the template path is hardcoded to the parent:
https://trac.buddypress.org/browser/tags/1.2.8/bp-themes/bp-default/functions.php#L19

Needs to be patched to use STYLESHEETPATH instead of TEMPLATEPATH.

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Regarding your problem, make sure you haven’t changed any of the list tab IDs. The AJAX requests rely on the IDs of the tabs.