To fix Internal Exception: java.io.IOException, An existing connection was forcibly closed by the remote host Minecraft error, you need to follow the given solutions.

Enabling the Java Native Sandbox option has helped some users clear up their issues related to io.netty.handler and Java.IO.IOException. Press the Windows key (or click the start menu icon) ...

The Internal Exception Java.IO.IOException error usually occurs when you launch Minecraft, or when the game is underway. This error can be caused by a number of different things.