Strange WCF net.tcp exception

I have a WCF client/service with net.tcp transport. When I turn on the WCF tracing on client side I am seeing the following errors in trace (see screenshot from service trace viewer). The strange thing is that WCF is handling and recovering this error and my client doesn't receive any exception and it continues to work. This exception happens freqently, randomly but not on every web method call. Аутентификация клиента (windows XP) - это windows, служба идентифицируется с помощью SPN, службы самостоятельно размещаются в службе windows за NLB (windows server 2003). Может кто-нибудь объяснить мне, что здесь происходит.

Трассировка стека исключений из трассировки xml:

<ExceptionString>
System.ServiceModel.Security.MessageSecurityException: The server rejected the upgrade request. ---&gt; System.ServiceModel.ProtocolException: Error while reading message framing format at position 0 of stream (state: ReadingUpgradeRecord) ---&gt; System.IO.InvalidDataException: More data was expected, but EOF was reached.
   --- End of inner exception stack trace ---
   --- End of inner exception stack trace ---
</ExceptionString>

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