Thank you very much for suggestions. The lesson learned is that DCOM communications can cause major headaches.
As you suspected, AX-S4 ICCP server in our environment was not running as a service.
After we made the change to run it as a service everything became happy again. And, since it will be run as a service in the production environment, I abandoned any further attempts to fix DCOM problems.
Thanks again.

I’m developing a service that needs to publish OPC data to SISCO AX-S4 ICCP server. The service is Windows server, which serves as wrapper for WCF service. Basically, all of the business logic is encapsulated in WCF service, and Windows service provides basic OS interaction (Start/Stop).

In its simplest form, the code needs to do read one value from the server, and to write one value to the server. Both the service and OPC server reside on the same physical box. Ability to read and write desired values has been verified through use of Matrikon and SISCO OPC clients… Firewall is turned off…

The following error comes back (including a part of my error message specifying parameters that get passed in), when the following call is made against SISCO AX-S4 ICCP server

_easyDAClient.WriteItemValue(_activeICCPLinkServer, _serverClass, itemName, itemValue);

“Unable to write to OPC server _activeICCPLinkServer =localhost, _serverClass=SISCO.AXS4ICCP.3, _itemName=UtilitySiteA:DataValue~Measurement_03.Value, _itemValue=0.3464102
2012-11-29 13:43:46.6427|INFO|PDAC.PdacToEMS.Service.Controller.DataProcessor|Publish Data Failure. Exception: OpcLabs.EasyOpc.OpcException: OPC operation failure. ---> System.Runtime.InteropServices.COMException: Server execution failed”

However, the same code works when it is run against Matrikon OPC Server. And, the most puzzling part for the moment, is that code works against both OPC servers when executed from a Windows Forms App, instead of a service.

Any help would be appreciated.