I think I understand how your setup is configured, but you really need to post more information if you expect anything but pure speculation. Your best chance to resolve this problem is through direct contact with Rockwell Automation and GE.
For those speculating along:
RSLinx Enterprise is the data engine that supplies connectivity between the FactoryTalk LiveData service and Rockwell Automation controllers. We can safely assume that's what
mbtoy is running.
FactoryTalk OPC Gateway is a relatively new (2 years) product that allows ordinary OPC clients (instead of FactoryTalk LiveData clients) to use RSLinx Enterprise.
Sheldn is probably referring to the notorious "DCOM 108" problem, which RA and Microsoft rooted out in Windows 2000 and 2003 Server operating systems about the same time Microsoft announced they were going to drop DCOM support from Windows Vista anyhow. I cannot rule this out. The repairs for "DCOM 108" are Windows patches, and the problem is well described in RA Knowledgebase
Answer ID # 37039.
I suggest two diagnostic approaches.
The first is to use the FactoryTalk Diagnostic Log, which SLC_Integrator recommended.
The second is to run the OPC Test Client, which is included with RSLinx Classic and probably also bundled with FactoryTalk OPC Gateway. This will tell you if all OPC connectivity has failed, or if it's just the connections between FactoryTalk OPC Gateway and your Cimplicity client.
My third suggestion is that you may not find the problem in the RSI software or in DCOM. In my experience with very large Cimplicity installations (at a certain microprocessor industry leader whose name rhymes with "Schmintel"), Cimplicity clients would disconnect themselves from running OPC Servers (both RA and others) and require reboot. That was about four years ago, but I don't think it's beyond possibility.