I've run into the same issue and filed a support ticket. Their answer was:
"Warning : caused by java.io.IOException: Broken pipe" usually means that the process wants to write data to a connection that is not available anymore. For example, this could happen when a request is done, the application starts processing the request, in the meanwhile the user closes their browser (or a web service call closes the HTTP connection), and when the application is done processing the request and wants to write the response back to the HTTP connection, it's not there anymore. This might also result in deadlocks.
It is recommended to check what it is running to find out the root cause for the issue. For that you can use the Runing Now Metrics (https://docs.mendix.com/developerportal/operate/troubleshooting-mxcloud-runningnow#1-introduction) and also you can use Query Logging (https://docs.mendix.com/refguide/tricky-custom-runtime-settings#3-query-logging).
Nikel, we have currently the same after a migration from Mx4 to Mx6. Did you ever find a sollution for this?
Regards,
Ronald
I had similar warnings in one of our apps. The warning was caused by the file document viewer (running MX7). Setting the configuration option "Use PDF js” solved the problem in my case.