There are a number of logs you can look at to try and gain some insight. They are located (by default) either in /var/log/mdce on Linux/Mac, and %TEMP%\MDCE\log on Windows. This might tell you what was going on.
Since the connection was "forcibly closed", that could be the result of something at the OS level, and you could take a look at the system logs / event viewer for any clues as to what might be going on.
Without reviewing the logs, though, there's not a lot to go on. There are many reasons something could forcibly shut down.
You can also try running a validation of the cluster (Parallel, Manage, select cluster profile, validate) or run the connectivity tests in Admin Center (matlabroot/toolbox/distcomp/bin/admincenter) to see if there's something off with respect to your setup.
Note I'm assuming that you are using MDCE. It would also be helpful if you could list what OS you are on, too.