The ls-proxy-adapters.jar library that contains RobustNetworkedMetadataProvider should also contain the version of NetworkedMetadataProvider that was compiled together with it.
So, it seems that your Server configuration contains both the old and the new version of ls-proxy-adapters.jar and the Server loads the old version of NetworkedMetadataProvider. May you please check?
Note that Lightstreamer Server does not offer ClassLoader separation between different Adapter set configurations.