The configuration system has already been initialized.

Hi
I have customer facing error as subject. However, the same app runing fine in our testing environment and also at the rest of customer.
Any ideas? Please advice. tq
Event Type: Error Event Source: ASP.NET 2.0.50727.0 Event Category: None Event ID: 1334 Date: 10/3/2013 Time: 12:16:48 PM User: N/A Computer: WDAOAAPPS02 Description: Failed to initialize the AppDomain:/LM/w3svc/1/Root/FlexEA Exception: System.InvalidOperationException Message: The configuration system has already been initialized. StackTrace: at System.Configuration.ConfigurationManager.SetConfigurationSystem(IInternalConfigSystem configSystem, Boolean initComplete) at System.Configuration.Internal.InternalConfigSettingsFactory.System.Configuration.Internal.IInter ...