I had the same issue with 1 user today - Blackberry was able to send and receive mail normally. User was unable to access mailbox via Outlook 2003 but was able to access via OWA. Administrators were able to access mailbox via Outlook 2003, but not with a dedicated Outlook profile.
Event 9646 was logged on Exchange 2003 SP2 server - Object type was "session".
KB842022 did not resolve the issue, but I left the registry setting in place.
Resolved issue by adding:
dword value - Maximum Allowed Sessions Per User
See Post by XMSRE in article Microsoft: Exchange Server 2003 - Event 9646 Exceeded maximum objects of type "session"
Be sure to restart Exchange Mailbox Store after adding the registry key