What causes the error ... ESB section is missing from the configuration source?

Question

We have an issue where connection between the BizTalk servers and the backing SQL Server is lost. Some of the orchestrations are exposed as WCF end points in IIS

When the SQL connection problem occurs, we get errors ...

The ESB section is missing from the configuration source.     
Source: Microsoft.Practices.ESB.Itinerary.PipelineComponents.ItinerarySelector     

The only fix seems to be doing an IISRESET, which is a bit heavy handed

Seems the ESB Toolkit uses the Enterprise Library ConfigurationSource to read the esb.config

But why is the ESB section being lost?

And even better, a real fix for the problem!