Sitecore 8 Analytics: PostSessionEndPipeline failed - mongodb

Question

I have recently configured MongoDB and Sitecore 8 Analytics (rev. 150812) on a production environment and it works just fine. However, oddly, I get this error in log files for every single pageview on live website, not sure why it exactly happens and how to rectify it. It is always a pair of errors (PostSessionEndPipeline failed + PostSessionEndPipeline failed). This happens on content delivery server. MongoDB is on another server. delivery server is on one Azure VM and MongoDB is on another VM. Does it have anything to do with session timeout or something like that?
7072 00:09:19 ERROR PostSessionEndPipeline failed.
Exception: System.IO.IOException
Message: Unable to read data from the transport connection: A connection attempt failed because the connected party did not properly respond after a period of time, or established connection failed because connected host has failed to respond.
Source: System
at System.Net.Sockets.NetworkStream.Read(Byte[] buffer, Int32 offset, Int32 size)
at MongoDB.Bson.IO.ByteBufferFactory.LoadFrom(Stream stream)
at MongoDB.Driver.Internal.MongoConnection.ReceiveMessage[TDocument](BsonBinaryReaderSettings readerSettings, IBsonSerializer serializer, IBsonSerializationOptions serializationOptions)
at MongoDB.Driver.Operations.QueryOperation`1.GetFirstBatch(IConnectionProvider connectionProvider)
at MongoDB.Driver.Operations.QueryOperation`1.Execute(IConnectionProvider connectionProvider)
at System.Linq.Enumerable.FirstOrDefault[TSource](IEnumerable`1 source)
at MongoDB.Driver.MongoCollection.FindOneAs[TDocument](FindOneArgs args)
at Sitecore.Analytics.Data.DataAccess.MongoDb.MongoDbCollection.<>c__DisplayClass1d`1.<FindOneByIdAs>b__1c()
at Sitecore.Analytics.Data.DataAccess.MongoDb.MongoDbCollection.Execute(Action action, ExceptionBehavior exceptionBehavior)
at Sitecore.Analytics.Data.DataAccess.MongoDb.MongoDbCollection.FindOneByIdAs[TDocument](BsonValue value, Nullable`1 exceptionBehavior)
at Sitecore.Analytics.Data.DataAccess.MongoDb.MongoDbDriver.FindOneByIdAs[T](Object key)
at Sitecore.Analytics.Data.DataAccess.MongoDb.MongoDbDictionary.LoadAs[T](Object key)
at Sitecore.Analytics.Data.Dictionaries.TrackingDictionary`2.Get(TKey key, LookupStrategy strategy)
at Sitecore.Analytics.Pipelines.EnsureClassification.GetClassificators.Process(EnsureClassificationPipelineArgs args)
at (Object , Object[] )
at Sitecore.Pipelines.CorePipeline.Run(PipelineArgs args)
at (Object , Object[] )
at Sitecore.Pipelines.CorePipeline.Run(PipelineArgs args)
at Sitecore.Analytics.Pipelines.PostSessionEnd.CommitSession.Process(PostSessionEndArgs args)
at (Object , Object[] )
at Sitecore.Pipelines.CorePipeline.Run(PipelineArgs args)
at Sitecore.Web.Application.RaiseSessionEndEvent(HttpApplication context)
Nested Exception
Exception: System.Net.Sockets.SocketException
Message: A connection attempt failed because the connected party did not properly respond after a period of time, or established connection failed because connected host has failed to respond
Source: System
at System.Net.Sockets.NetworkStream.Read(Byte[] buffer, Int32 offset, Int32 size)
1572 00:09:19 ERROR SessionEndPipeline failed.
Exception: System.IO.IOException
Message: Unable to read data from the transport connection: A connection attempt failed because the connected party did not properly respond after a period of time, or established connection failed because connected host has failed to respond.
Source: System
at System.Net.Sockets.NetworkStream.Read(Byte[] buffer, Int32 offset, Int32 size)
at MongoDB.Bson.IO.ByteBufferFactory.LoadFrom(Stream stream)
at MongoDB.Driver.Internal.MongoConnection.ReceiveMessage[TDocument](BsonBinaryReaderSettings readerSettings, IBsonSerializer serializer, IBsonSerializationOptions serializationOptions)
at MongoDB.Driver.Operations.QueryOperation`1.GetFirstBatch(IConnectionProvider connectionProvider)
at MongoDB.Driver.Operations.QueryOperation`1.Execute(IConnectionProvider connectionProvider)
at System.Collections.Generic.List`1..ctor(IEnumerable`1 collection)
at System.Linq.Enumerable.ToList[TSource](IEnumerable`1 source)
at Sitecore.Analytics.Automation.Data.MongoDbAutomationProvider.GetAutomationDocuments(ID contactId, IReadOnlyCollection`1& stateDocuments, IReadOnlyCollection`1& transitionDocuments)
at Sitecore.Analytics.Automation.Data.MongoDbAutomationProvider.GetAutomationContext(ID contactId, DateTime timestampExclusive)
at Sitecore.Analytics.Automation.Data.AutomationProvider.GetAutomationContext(ID contactId)
at Sitecore.Analytics.Automation.Data.AutomationStateManager.Create(Contact contact)
at Sitecore.Analytics.Automation.ContactAutomationEvents.Run(Session session, PageEventItem pageEventDefinitionItem)
at (Object , Object[] )
at Sitecore.Pipelines.CorePipeline.Run(PipelineArgs args)
at (Object , Object[] )
at Sitecore.Pipelines.CorePipeline.Run(PipelineArgs args)
at Sitecore.Web.Application.RaiseSessionEndEvent(HttpApplication context)
Nested Exception
Exception: System.Net.Sockets.SocketException
Message: A connection attempt failed because the connected party did not properly respond after a period of time, or established connection failed because connected host has failed to respond
Source: System at System.Net.Sockets.NetworkStream.Read(Byte[] buffer, Int32 offset, Int32 size)