You run Exchange Server 2010 and Exchange Server 2013 in a coexistence organization. When you use Outlook Web App (OWA) web parts to access a delegated calendar, you can't access to the calendar, and you receive the following error message:
"The Custom error module does not recognize this error "The Custom error module does not recognize this error".
To fix this issue, install Cumulative Update 11 for Exchange Server 2013.
Microsoft has confirmed that this is a problem in the Microsoft products that are listed in the "Applies to" section.