In the past I’ve often seen misconfigured NTFS rights cause OAB download fails in Outlook 2007 and 2010.
In a standard installation of Exchange 2010 the OAB’s physical directory points to:
C:\Program Files\Microsoft\Exchange Server\V14\ClientAccess\OAB\GUID-XXXXX-YYYYYY-ZZZZZ
You’ve to controll if the “Authenticated Users” group has permissions to read on your OAB directory (-> right click GUID-XXXXX-YYYYYY-ZZZZZ -> security) and if this right is inherited.
Cheers,
Chris