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Internal Exception

Connections drop with an IOException saying the connection was forcibly closed. Something outside Minecraft, a firewall, antivirus, or an unstable network, is cutting the connection mid-stream.

What does this error mean?
This error means the TCP connection was terminated unexpectedly by the other end or the network. Minecraft itself is usually healthy, the interruption comes from security software, packet loss, or a flaky link. 
java.io.IOException: An existing connection was forcibly closed
Most Common Causes
  • Antivirus or firewall closing the connection.
  • Packet loss or an unstable network path.
  • An overly aggressive anti-DDoS/proxy layer.
  • VPN or proxy interference.
  • Corrupted or oversized packets from a misbehaving mod/plugin.
How To Diagnose
  1. Determine if it affects one player or many.
  2. Temporarily disable antivirus/firewall on the affected side to test.
  3. Run a ping/MTR to check for packet loss.
  4. Note whether a specific action (opening a GUI, a plugin) triggers it.
Recommended Fixes
  • Whitelist in security software
    Add Java/the server to firewall and antivirus exceptions on the affected machine.
  • Fix the network
    Resolve packet loss, switch off a problematic VPN, or improve the connection.
  • Review the proxy layer
    If using a proxy or anti-DDoS, ensure it is not severing connections.
  • Check mods/plugins
    If a specific feature triggers it, update or remove the mod/plugin sending bad packets.
Analyze My Log
Frequently Asked Questions

No, this is a network/connection error, not world corruption.

Some security tools inspect or block game traffic and close the connection.

Their local network or security software is likely interfering, the server is fine for others.