We have a dev -> staging -> production environment and Courier used to work well but at some point it started failing mainly not transferring media files. Once the media files fail everything else fails.

We don't have SSL or anything, just plain old HTTP.

We are getting the following in the courier log:

2016-10-18 14:42:17,919 [21] ERROR Umbraco.Courier.Core.Diagnostics.Logging.RevisionLog - [Thread 30] [Error]; Item Id: e73991f1-574b-4eee-876b-011a355a0488; Caller: Umbraco.Courier.RepositoryProviders.CourierWebserviceRepositoryProvider; Process: resources; Msg: System.Net.WebException: The underlying connection was closed: An unexpected error occurred on a receive. ---> System.IO.IOException: Unable to read data from the transport connection: An existing connection was forcibly closed by the remote host. ---> System.Net.Sockets.SocketException: An existing connection was forcibly closed by the remote host
   at System.Net.Sockets.Socket.Receive(Byte[] buffer, Int32 offset, Int32 size, SocketFlags socketFlags)
   at System.Net.Sockets.NetworkStream.Read(Byte[] buffer, Int32 offset, Int32 size)
   --- End of inner exception stack trace ---
   at System.Net.Sockets.NetworkStream.Read(Byte[] buffer, Int32 offset, Int32 size)
   at System.Net.Connection.SyncRead(HttpWebRequest request, Boolean userRetrievedStream, Boolean probeRead)
   --- End of inner exception stack trace ---
   at System.Web.Services.Protocols.WebClientProtocol.GetWebResponse(WebRequest request)
   at System.Web.Services.Protocols.SoapHttpClientProtocol.Invoke(String methodName, Object[] parameters)
   at Umbraco.Courier.RepositoryProviders.WebServiceProvider.RepositoryWebservice.TransferResources(String sessionKey, ResourceTransfer[] resources, String user, String pass)
   at Umbraco.Courier.RepositoryProviders.CourierWebserviceRepositoryProvider.TransferResources(ResourceTransfer[] resources);
System.Exception: Error logged

Additionally, when I see the browser (Chrome dev tools), I see a ton of requests to a web service and when it fails I get:

{"Message":"Client found response content type of \u0027text/xml; charset=utf-8\u0027, but expected \u0027text/xml\u0027.\r\nThe request failed with an empty response.","StackTrace":"   at Umbraco.Courier.Core.TaskManager.GetSuccessfulProcessedTasks()\r\n   at Umbraco.Courier.UI.Pages.ViewTaskManager.GetProcessed()","ExceptionType":"System.InvalidOperationException"}

I have tried different things, including increasing the maxDataLength and executionTimeout parameters in the httpRuntime section of the web.config file.

Does anyone knows what could be causing this?