Active sync issue when published via MS Web Application Proxy

Exchange 2013 CU7

If you are getting this error on the CAS servers

[Eas] The number of outstanding requests for guard TargetBackend(“servername”) has exceeded the max limit 150. Current request will be rejected.

And this error on the Microsoft Web Application Proxy

The HTTP response from the back end server was not received within the expected interval. Expected interval: 300 seconds.

• Details:
  Transaction ID: {f4be1653-43e9-0000-02d0-bef4e943d001}
  Session ID: {f4be1653-43e9-0000-02d0-bef4e943d001}
  Published Application Name: Exchange Microsoft-Server-ActiveSync
  Published Application ID: 9ad23d25-1d98-7a66-3671-6cd7aabcab29
  Published Application External URL: https://mailserver.address.com/Microsoft-Server-ActiveSync/
  Published Backend URL: https://mailserver.address.com/Microsoft-Server-ActiveSync/
  User: <Unknown>
  User-Agent: Apple-iPhone5C2/1104.257
  Device ID: <Not Applicable>
  Token State: NotFound
  Cookie State: NotFound
  Client Request URL: https://mailserver.address.com/Microsoft-Server-ActiveSync?User=user@address.com&DeviceId=ApplF2LL1VVXDTWF&DeviceType=iPhone&Cmd=Ping
  Backend Request URL: https://mailserver.address.com/Microsoft-Server-ActiveSync?User=user@address.com&DeviceId=ApplF2LL1VVXDTWF&DeviceType=iPhone&Cmd=Ping
  Preauthentication Flow: PassThrough
  Backend Server Authentication Mode: PassThrough
  State Machine State: BEBodyWriting
  Response Code to Client: <Not Applicable>
  Response Message to Client: <Not Applicable>
  Client Certificate Issuer: <Not Found>

Try

Reference here 

• On all CAS servers:

  In \Exchange Server\V15\FrontEnd\HttpProxy\sync\web.config, add this key in the <appSettings> node.

  <add key=”HttpProxy.ConcurrencyGuards.TargetBackendLimit” value=”5000″ />

  In \Exchange Server\V15\FrontEnd\HttpProxy\rpc\web.config, add this key in the <appSettings> node.

  <add key=”HttpProxy.ConcurrencyGuards.TargetBackendLimit” value=”5000″ />

  After this change recycle the MSExchangeSyncAppPool and MSExchangeRPCProxyAppPool in IIS manager.

  After this, we stopped getting Event ID 2002, and all Connection_Dropped_Event_List_Full stopped appearing in Httperror logs.