BuddyPress.org

@Mohit Kumar: you are missing the cookie hash, etc. in your wp-config.php in addition to the lines I told you to add to your bb-config.php. Install the plugin https://wordpress.org/extend/plugins/bbpress-integration/

to wpmu and add the code it tells you to to your wp-config.

@Nicholaszen: is your wpmu site disappearing or your bbpress site? or both?