Type XXX was not assignable to 'com.google.gwt.user.client.rpc.IsSerializable' and did not have a custom field serializer. For security purposes, this type will not be deserialized."

We have one GWT application running on several servers. For one server
we get the message

Type XXX was not assignable to
'com.google.gwt.user.client.rpc.IsSerializable' and did not have a
custom field serializer. For security purposes, this type will not be
deserialized."

And on the other three servers it works fine. Does anyone know what
the problem can be? It is the exactly the same codebase located on the
different servers.

We use gwt.version 2.1.0.

Thanks,

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