Connection Error to iHistorian when CAT5 cable disconnected

I have a HMI station, with iFix SCADA Developer v5.5 and iHistorian v5.0 installed on the same computer.
When network is working correctly, I have no issues.
If the network goes down, or the CAT5 cable is not connected, I can connect to iHistorian using iHistorian Admin tool.
iHistorian service is running, iFix is running, and iFixCollector is running.

Does anybody know why I have this issue?

Thanks,
Philippe