What channel were you using before?
Can you successfully do a Clone() of your object graph?
It sounds like the message was not successfully or completely delivered over the wire. Either it was corrupted somehow, or truncated.
There are data and serialization size limits in WCF, and you may have to raise those limits through client and server configuration. The FAQ has pointers to some information around WCF configuration
http://www.lhotka.net/cslanet/faq/DataPortalFaq.ashx
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