When this problem occurs, the following entry is logged in the ActiveSync log:
Log Name: ApplicationNote This issue only occurs in Exchange Server 2013 Cumulative Update 6 with Exchange Server 2007 coexistence environment.
Source: MSExchange Common
Date: 9/2/2014 12:16:40 PM
Event ID: 4999
Task Category: General
Watson report about to be sent for process id: 1948, with parameters: E12, c-RTL-AMD64, 15.00.0995.029, w3wp#MSExchangeSyncAppPool, M.Exchange.AirSyncHandler, M.E.A.Handler.LegacyUserAllowedToSync, System.InvalidCastException, 9619, 15.00.0995.026.