Bad request on Push Session

I have been seeing a sequence of error logs in my pre-production environment. (One CM and 2 CD with netscaler).

The sequence is like this:

592 00:00:00 WARN Sitecore.Support.147407: aborting StartAnalytics pipeline since Tracker.Current is not initialized.
6220 00:00:00 WARN Binding for type Sitecore.Foundation.SitecoreExtensions.Facets.CustomMemberData from assembly Sitecore.Foundation.SitecoreExtensions, Version=, Culture=neutral, PublicKeyToken=null is not allowed.
7320 00:00:00 ERROR Cannot push session url: preprodaddress/.../PushSession.ashx
7320 00:00:00 ERROR Cannot create tracker.
Exception: System.Net.WebException
Message: The remote server returned an error: (400) Bad Request.
Source: System
at System.Net.WebClient.UploadDataInternal(Uri address, String method, Byte[] data, WebRequest& request)
at System.Net.WebClient.UploadData(Uri address, String method, Byte[] data)
at Sitecore.Analytics.Pipelines.TransferSession.TransferSessionToDifferentCluster.UploadData(Uri url, Byte[] data)
at Sitecore.Analytics.Pipelines.TransferSession.TransferSessionToDifferentCluster.PushSession(TransferSessionArgs args)
at Sitecore.Analytics.Pipelines.TransferSession.TransferSessionToDifferentCluster.Process(TransferSessionArgs args)
at (Object , Object[] )
at Sitecore.Pipelines.CorePipeline.Run(PipelineArgs args)
at Sitecore.Analytics.Pipelines.TransferSession.TransferSessionPipeline.Run(TransferSessionArgs args)
at Sitecore.Analytics.Pipelines.EnsureSessionContext.ClusterCheck.Process(InitializeTrackerArgs args)
at (Object , Object[] )
at Sitecore.Pipelines.CorePipeline.Run(PipelineArgs args)
at Sitecore.Analytics.Pipelines.EnsureSessionContext.EnsureSessionContextPipeline.Run(InitializeTrackerArgs args)
at Sitecore.Analytics.DefaultTracker.EnsureSessionContext()
at Sitecore.Analytics.Pipelines.CreateTracker.GetTracker.Process(CreateTrackerArgs args)
at (Object , Object[] )
at Sitecore.Pipelines.CorePipeline.Run(PipelineArgs args)
at Sitecore.Analytics.Tracker.Initialize()

It kind of repeats over and over. Not sure why the tracker is not initialized. This is a log from the CD, I am not seeing this error in CM. I should also add that I am not seeing issues with the analytics. But every now and then, my custom contact facets suffer sudden changes that are hard to explain, and I am looking at this bug as a possible culprit.

This environment also has a default proxy on the CM to allow it to connect to the CD. (this is different from our other environments).

Any ideas what this might mean?

Thanks in advance!