I did setup a 6.0.1 GA using our 5.0.16 NE as external authentication source for users.
During the setup process everything works fine - including making a successful test on a user bind before saving the setup:

When I then create eaxctly the same user used for testing I am not able to login.
Still I am able to 'View' the mailbox from the Admin UI (preauth).

Any ideas?