The remote RPC to the application server failed for the call datastore_v3.Put()

Question

I'm using Google App Engine, with Java, Managed VM and maven-gcloud-plugin.

When I run my app locally (mvn gcloud:run) I get random exceptions when I write to the Datastore (via Objectify) saying:

The remote RPC to the application server failed for the call datastore_v3.Put()

My code doesn't do much more than that

public class TestDAOImpl implements TestDAO {
    @Override
    public void save(Test test) {
        ofy().save().entity(test).now();

    }
}

On the actual App Engine instance, everything seems to run fine though...

Does anyone know why is that happening and how to prevent it?

Thanks