I am running WP 3.4 and BP 1.5.6. (Non multi-install).
If I request a member's page of a user which does not exist (http://domain.com/buddypress/members/xxx/), I get the following error:
Fatal error: Call to a member function set_404() on a non-object in /www/buddypress/wp-content/plugins/buddypress/bp-core/bp-core-functions.php on line 1470
I found a similar post here, but without a solution:
Can someone try and reproduce the error by attempting to access a non-existant user? (http://domain.com/buddypress/members/xxx/)