Hi,

Im running Exchange server 2010 SP3 with a casarray (2 nodes) and DAG (2 nodes).

the only particular configuration is OWA/EWS external URL is empty as it is only used internally. It has being working for the past 2 years since the migration from Exchange server 2007 was completed.

I have a very strange behavior on my DAG node:

2 months ago, node1 was login the error 4002. it started from the noting, stay there from a month an went away on it's own.

I dug every setting I could find and nothing was misconfigured.

now the second node started login the same error:

Process 3216: ProxyWebRequest CrossSite from S-1-1-0 to https://myowa.mydomain.local/EWS/exchange.asmx failed. Caller SIDs: NetworkCredentials. The exception returned is Microsoft.Exchange.InfoWorker.Common.Availability.ProxyWebRequestProcessingException: System.Net.WebException: The request was aborted: The request was canceled.
   at System.Web.Services.Protocols.WebClientAsyncResult.WaitForResponse()
   at System.Web.Services.Protocols.WebClientProtocol.EndSend(IAsyncResult asyncResult, Object& internalAsyncState, Stream& responseStream)
   at System.Web.Services.Protocols.SoapHttpClientProtocol.EndInvoke(IAsyncResult asyncResult)
   at Microsoft.Exchange.InfoWorker.Common.Availability.Proxy.Service.EndGetUserAvailability(IAsyncResult asyncResult)
   at Microsoft.Exchange.InfoWorker.Common.Availability.FreeBusyApplication.EndProxyWebRequest(ProxyWebRequest proxyWebRequest, QueryList queryList, Service service, IAsyncResult asyncResult)
   at Microsoft.Exchange.InfoWorker.Common.Availability.ProxyWebRequest.EndInvoke(IAsyncResult asyncResult)
   at Microsoft.Exchange.InfoWorker.Common.Availability.AsyncWebRequest.EndInvokeWithErrorHandling():<No response>. The request information is ProxyWebRequest type = CrossSite, url = https://myowa.mydomain.local/EWS/exchange.asmx
Mailbox list = <user>SMTP:user@myowa.mydomain.local, Parameters: windowStart = 01/02/2015 00:00:00, windowEnd = 01/04/2015 00:00:00, MergedFBInterval = 30, RequestedView = FreeBusy
. ---> System.Net.WebException: The request was aborted: The request was canceled.
   at System.Web.Services.Protocols.WebClientAsyncResult.WaitForResponse()
   at System.Web.Services.Protocols.WebClientProtocol.EndSend(IAsyncResult asyncResult, Object& internalAsyncState, Stream& responseStream)
   at System.Web.Services.Protocols.SoapHttpClientProtocol.EndInvoke(IAsyncResult asyncResult)
   at Microsoft.Exchange.InfoWorker.Common.Availability.Proxy.Service.EndGetUserAvailability(IAsyncResult asyncResult)
   at Microsoft.Exchange.InfoWorker.Common.Availability.FreeBusyApplication.EndProxyWebRequest(ProxyWebRequest proxyWebRequest, QueryList queryList, Service service, IAsyncResult asyncResult)
   at Microsoft.Exchange.InfoWorker.Common.Availability.ProxyWebRequest.EndInvoke(IAsyncResult asyncResult)
   at Microsoft.Exchange.InfoWorker.Common.Availability.AsyncWebRequest.EndInvokeWithErrorHandling()
   --- End of inner exception stack trace ---
. Name of the server where exception originated: MSX02. Make sure that the Active Directory site/forest that contain the user's mailbox has at least one local Exchange 2010 server running the Availability service. Turn up logging for the Availability service and test basic network connectivity.

there were no changes that could have triggered the error. And so far the users did not complain about issues.

Also the LB is done by F5 BigIp

I run out of ideas, and settings to check :(

Please advise

thanks!