BuddyPress.org

This problem has been reported by sites running IIS.

Try this solution:

https://buddypress.org/forums/topic/seeing-page-not-found-on-activity-members-groups-blogs-tabs#post-46478

You could also try this patch:

https://trac.buddypress.org/ticket/2299

Or try this plugin as well (if you have ISAPI Rewrite 3.0 enabled on your IIS server):

https://wordpress.org/extend/plugins/isapi-rewriter/