Nginx is logging that error because the upstream, in this case your mendix runtime process, is dropping the request without closing the HTTP connection properly. There could be several reasons why this is happening. I advise you to take a look at the monitoring graphs. In particular the JVM threads, JVM heap and other memory metrics.
As suggested by Andrej probably best to reach out to Mendix Support because you are running in the Mendix Cloud (looks like it).