In a Microsoft Exchange Server 2013 environment, the OWA application pool crashes several times a day. In this situation, the following event is logged:
This issue occurs because Exchange Server expects all meeting participants to be rendered. However, for some special meetings, some participants may not be rendered successfully. When this issue occurs, a KeyNotFound exception is displayed, and the OWA application pool crashes.
Microsoft has confirmed that this is a problem in the Microsoft products that are listed in the "Applies to" section.