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.

Java exception handling with stack traces, exception chaining, try-with-resources, final re-throw, and StackWalker.

If you use any Java internal APIs in your code, it is time to stop. And if you don't stop on your own, Oracle will stop it for you.