This can happen when NGINX started a request to uWSGI but uWSGI took too long to respond, then NGINX closes the connection to uWSGI. When uWSGI finally finishes, it tries to give it’s response back to NGINX, but NGINX closed the connection earlier, so then uWSGI throws an I/O error.
So this could mean that your uWSGI process is taking too long.
Update:
uWSGI’s Harakiri mode could be useful to automatically terminate such long taking processes if you want to:
https://uwsgi-docs.readthedocs.io/en/latest/FAQ.html#what-is-harakiri-mode
You might not want to do this because a process might be finishing some long query or something which could be necessary.