Event ID 1003: MSExchangeFront End HTTP Proxy ([Oab] An internal server error occurred.)

Every 10 minutes or so i get the following two Events recorded at the same time in the system event log.  Anyone know what they point to?  OAB seems healthy.

Event ID 1003 MSExchange Front End HTTP Proxy
[Oab] An internal server error occurred. The unhandled exception was: System.ArgumentNullException: Value cannot be null.
Parameter name: sourceObject
   at Microsoft.Exchange.HttpProxy.AnchorMailbox..ctor(AnchorSource anchorSource, Object sourceObject, IRequestContext requestContext)
   at Microsoft.Exchange.HttpProxy.UserADRawEntryAnchorMailbox..ctor(ADRawEntry adRawEntry, IRequestContext requestContext)
   at Microsoft.Exchange.HttpProxy.OabProxyRequestHandler.ResolveAnchorMailbox()
   at Microsoft.Exchange.HttpProxy.ProxyRequestHandler.InternalBeginCalculateTargetBackEnd(AnchorMailbox& anchorMailbox)
   at Microsoft.Exchange.HttpProxy.ProxyRequestHandler.<BeginCalculateTargetBackEnd>b__27()
   at Microsoft.Exchange.Common.IL.ILUtil.DoTryFilterCatch(TryDelegate tryDelegate, FilterDelegate filterDelegate, CatchDelegate catchDelegate)


Event ID 1309 ASP.NET 4.0.30319.0

Event code: 3005 
Event message: An unhandled exception has occurred. 
Event time: 8/13/2013 2:00:35 PM 
Event time (UTC): 8/13/2013 8:00:35 PM 
Event ID: b30166423b674320a9304ba6528b18e5 
Event sequence: 3580 
Event occurrence: 1193 
Event detail code: 0 
 
Application information: 
    Application domain: /LM/W3SVC/1/ROOT/OAB-1-130208036285056837 
    Trust level: Full 
    Application Virtual Path: /OAB 
    Application Path: D:\Microsoft\Exchange Server\V15\FrontEnd\HttpProxy\OAB\ 
    Machine name: ICTEXCH01 
 
Process information: 
    Process ID: 20368 
    Process name: w3wp.exe 
    Account name: NT AUTHORITY\SYSTEM 
 
Exception information: 
    Exception type: ArgumentNullException 
    Exception message: Value cannot be null.
Parameter name: sourceObject
   at Microsoft.Exchange.HttpProxy.ProxyRequestHandler.EndProcessRequest(IAsyncResult result)
   at System.Web.HttpApplication.CallHandlerExecutionStep.OnAsyncHandlerCompletion(IAsyncResult ar)

Request information: 
    Request URL: https://mail.contoso.net:443/OAB/25da1865-3771-44fc-9373-3f08467734b1/oab.xml 
    Request path: /OAB/25da1865-3771-44fc-9373-3f08467734b1/oab.xml 
    User host address: 172.16.14.188 
    User: contoso\username 
    Is authenticated: True 
    Authentication Type: Negotiate 
    Thread account name: NT AUTHORITY\SYSTEM 
 
Thread information: 
    Thread ID: 13 
    Thread account name: NT AUTHORITY\SYSTEM 
    Is impersonating: False 
    Stack trace:    at Microsoft.Exchange.HttpProxy.ProxyRequestHandler.EndProcessRequest(IAsyncResult result)
   at System.Web.HttpApplication.CallHandlerExecutionStep.OnAsyncHandlerCompletion(IAsyncResult ar)