BuddyPress.org

I re-read your post. There have been noted problems with IIS and BuddyPress.

Here are some solutions you can try:
https://trac.buddypress.org/ticket/2299
https://buddypress.org/community/groups/how-to-and-troubleshooting/forum/topic/seeing-page-not-found-on-activity-members-groups-blogs-tabs/
http://www.matthewcasey.co.uk/2010/04/01/setting-up-wordpress-mu-on-iis/