If you try and IM a user who is using Exchange 2010 OWA with OCS integration and receive a ‘reference error ID 504’ error then don’t despair.

You will probably find that the user can send IM out but you can’t reply.

The resolution is to simply apply update UcmaRedist.msp on your Exchange 2010 CAS Servers:

http://www.microsoft.com/downloads/en/details.aspx?displaylang=en&FamilyID=b3b02475-150c-41fa-844a-c10a517040f4

Good luck

Regards

Neil