Maximum number of items that can be serialized or deserialized in an object graph... with knowtypes

Don't forget to check client configuration.

See similar answers in How to fix MaxItemsInObjectGraph error?

You need to set the MaxItemsInObjectGraph on the dataContractSerializer using a behavior on both the client and service.

and maxItemsInObjectGraph ignored

I had forgot to place this setting in my client app.config file

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