Fix "Internal Exception" on Minecraft

A player is dropped to a Connection Lost screen (Failed to connect to the server if it happens while joining) whose text starts with Internal Exception: followed by a long Java error. The words after "Internal Exception" are the only real clue. The sections below read the error, separate a server fault from a connection fault, and list what to change.

Checked against Minecraft 26.3 (Mojang's own game files), October 2026.

What "Internal Exception" is

Minecraft shows Internal Exception: in front of a network error it has no friendlier message for. A connection that goes quiet for too long shows Timed out instead, and the game has separate texts for End of stream and Network Protocol Error. Most other network errors land here, and the text after the colon names the exception that broke the connection. Common ones:

• An existing connection was forcibly closed by the remote host. This is Windows' wording for a connection reset. Microsoft says it happens when the program on the other end is suddenly stopped, the other computer is rebooted, or the other side closes the connection abruptly.
• An established connection was aborted by the software in your host machine. Windows closed the connection on the player's own computer, which Microsoft says can follow a data transmission time-out or a protocol error. Look at the player's own computer first.
• io.netty.handler.codec.DecoderException. Netty, the library that carries Minecraft's network traffic, raises this when the data that arrived could not be decoded. A CorruptedFrameException is the same fault when a whole frame was unreadable. The text after it, and any mod or plugin name in it, tell you where to look.
• Server closed is not an exception at all: it is the message the server sends every connected player when it shuts down, so it appears when the server is stopped or restarted while the player is on.

Find out what happened on the server

1. Open the Console tab. If the disconnect just happened, the line is near the bottom. For older ones press Download Log, pick Last hour, and search the file.
2. Look for <player> lost connection: followed by a reason, and note the time of the line.
3. Check whether only one player dropped or everyone did at the same moment.
4. Scroll up from the line and look for Can't keep up! Is the server overloaded? Running <N>ms or <N> ticks behind, or, in the live Console, a red Your server crashed - restarting automatically…. A downloaded log keeps the raw lines instead, so look there for Exit code:.

If everyone dropped together

Our Can't connect to your game server? Fix it guide starts with the player's own connection when everyone else gets in and one player can't. When several players drop together that does not fit, so look at the server first. A crash line points you to Fix "Exception in server tick loop" crash or Minecraft server exit codes 1, 137 and 143. Repeated Can't keep up! lines point at lag, so work through Why is my server lagging? Find the cause. The TPS reading in the left rail shows whether it is struggling right now: it reads about 20 when the server is healthy.

If one player dropped

That player's connection or game is the likelier cause, so ask them to try these in order:

1. Close any VPN and rejoin.
2. Try another network, such as a phone hotspot. School, work and public Wi-Fi often interfere with game traffic.
3. Check whether a firewall or antivirus app on their computer is blocking game traffic.
4. Measure their ping to the server using the steps for measuring your own ping. A high or jumpy ping points at their connection.

If it started after adding a mod or plugin

1. Open the Mods tab (Plugins on Paper, Purpur and Folia) and switch to Installed.
2. Press Disable on the add-on you added last. The panel reminds you to restart the server to apply it.
3. Restart from the Console tab and have the player rejoin.

If the error stops, update that add-on from the same tab with Check for updates, or leave it off. Players on a Fabric, Forge or NeoForge server also need the same mods on their own game, as Join your Minecraft server (and fix errors) explains.