BuddyPress.org

Do you have ms defines in wp-config ?
define( 'WP_ALLOW_MULTISITE', true );

Have you setup pretty permalinks (something other than default) ? Also htaccess may be wrong.
Create a new one.
This can be done by removing the old htaccess and (re)saving permalinks, WP will give you new params or create htaccess for you.
Also are all salt keys in wp-config ? Try to remove the existing and add new one. Take them from here: https://api.wordpress.org/secret-key/1.1/salt/