Uppercase or Mixcase usernames problem: Greenhopper Assignee Board shows counts for only some users

The GH Assignee Board shows counts and sums for only some users (see ScreenshotGHAssignee.jpg).

We are using a Microsoft Active Directory as User Directory. GH only shows information from users that are created in AD in lowecase. Mixcase or Uppercase users don't have the information displayed on the board.