I'm getting these now too. Anyone?
In the Mendix cloud, all connections to Jetty are routed via nginx. So if stuff like this shows up in your log in the Mendix cloud, this indicates nginx is configured wrong. Ronald's assumption is probably right(ish). But whatever the client does, this should not make nginx drop the connection to Jetty without properly closing it.
If I'm wrong here please enlighten me, I am no nginx expert :).