BuddyPress.org

It has to do with your .htaccess. Are you sure you put the module code listed in the correct file??

There are several other solutions that may fit you. All it takes is some researching

https://buddypress.org/community/groups/how-to-and-troubleshooting/forum/topic/seeing-page-not-found-on-activity-members-groups-blogs-tabs/#post-46478

https://buddypress.org/community/groups/how-to-and-troubleshooting/forum/topic/seeing-page-not-found-on-activity-members-groups-blogs-tabs/#post-69444

http://premium.wpmudev.org/forums/topic/page-not-found-error-404

https://buddypress.org/community/groups/installing-buddypress/forum/topic/the-requested-url-register-was-not-found-on-this-server/

Also, I went on the site listed, and it seems you did something to change the permalinks. For example, your activity is linked to %site%/wp/activity??

If you changed permalinks, you must update the .htaccess file.