The only way I can replicate the problem is if I have two clients where one client does not have the correct auto serializer configuration. If the first request goes through the client which works and,say creates an object, then the second request goes to the misconfigured client, tries to retrieve the object and fails.
The failure is happening on pdx serializer's 'fromData' method. This would mean that a PDX object has already been successfully created somewhere but, now, cannot be deserialized.
I could see this happening where your client is embedded in a web application and you have a load balancer fronting multiple app servers, each running your web app. Then, if you don't have sticky sessions your requests would be round-robined across different app servers, triggering this problem.
I had already pointed out a misconfigured client-cache.xml, however you said fixing that didn't help. Perhaps just double check that your configuration is correct everywhere,
--Jens