Internal Exception: java.io.IOException: An existing connection was forcibly closed by the remote host

Booted from the server after 5 minutes. Was not inactive. idle-server-timeout was set to 60 (minutes), but I was only on the server for a few minutes. Changed idle-server-timeout to 0, same issue.

I've been booted from servers for actual inactivity before. There's an appropriate error message for that. I've been all over different forums, etc, a no one seems to know what causes this. I've also had this problem with other minecraft versions in the past when using different computers / operating systems.

What are known causes for this error?