Error code

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

MINECRAFTInternal Exception: java.io.IOException: An existing connection was forcibly closed by the remote host
Game/Platform Error· An error code shown by a game or gaming platform/launcher.

What it means

The remote server forcibly terminated your active connection.

How to fix it

  1. Restart your modem and router.
  2. allow Minecraft through Windows Firewall for private and public.
  3. update your network and graphics drivers.
  4. flush your DNS cache.
  5. test on vanilla Minecraft without mods.
  6. reinstall Java and Minecraft.

Not affiliated with Minecraft. Answer summarized from the manufacturer's manual linked above; consult that source for the authoritative procedure.