Try running an IEMSTest.exe on the BES as the BES Admin and post the results:
Should look something like this ...
C:\Program Files\Research In Motion\BlackBerry Enterprise Server\Utility>IEMSTest.exe
BlackBerry Enterprise Server Utility - IEMSTest.exe (IExchangeManageStore), Version 1.0
Copyright (c) Research In Motion, Ltd. 1999. All rights reserved.
Opening Default Message Store Mailbox - BES Admin.
Opening message store for Test Account using /o=MSG1 ORG/ou=First Administra
tive Group/cn=Recipients/cn=testacct1830 /o=MSG1 ORG/ou=First Administrative G
roup/cn=Configuration/cn=Servers/cn=MAILBOX101/cn=Microsoft Private MDB.
Test Account's Mailbox opened successfully.
Root Folder opened successfully.
Folder created successfully.
Test folder deleted successfully.
Test completed successfully for Test Account.
Is the user you're trying to activate in any "Special" Groups