Today I had a weird problem. Our BuddyPress activity stream comments were just not working. When you clicked the reply icon, then typed a response and pressed “post”, it simply grayed out and never succeeded. I could see, in the logs, that there was a 403 Forbidden error being thrown.
I just debugged this and the answer is quite simple.
In your theme, somewhere, there should be a /buddypress/activity/entry.php file.
In there, you might have something like:
<?php wp_nonce_field( 'new_activity_comment', '_wpnonce_new_activity_comment' ); ?>
This is wrong. Change it to:
<?php wp_nonce_field( 'new_activity_comment', '_wpnonce_new_activity_comment_' . bp_get_activity_id() ); ?>
And all will be well.