I posted about this in the past, and have 95% resolved it. We have 7
locations and Messenger is running on one server (just updated it to 2.0.1)
- all users from all sites/contexts are found just fine using any method -
except for ONE site, and any user within any context within that
organization. All contexts for all locations are in the default scope, and
work fine whether using basic, advanced, name, user ID, no matter, except
the one site.

The error when I try to pull one of those users into my messenger client
(using user ID method) is: "Requested user ID not found in the directory"

Any thoughts, or is any further information needed?