User unable to login with "unable to create user" message

I’m using this plugin in a WP multi-site install. A user who tries to login receives “Error creating user!” That same user, in the “Test Tool” I get this:

[NOTICE] Creating user ‘waterstc’ with following data:
– email : waterstc@pitt.edu
– first name : Thomas
– last name : Waters
– display name : WATERSTC
– account suffix: @pitt.edu
– role : editor
[DEBUG] Setting local password to the used for this login.
[ERROR] Sorry, that email address is already used!
[NOTICE] – user_id :
[FATAL] Error creating user.
Error creating user!

Is the issue surrounding the multisite set up?

http://wordpress.org/plugins/active-directory-integration/