Minecraft server errors, and how to fix them

1Errors, by what you see

Players can't connect, or get dropped

• Connection refused, as io.netty's AnnotatedConnectException: nothing takes connections at that address and port. The same page covers connection timed out.
• Timed out: a player in the game and the server stopped hearing from each other.

Players are turned away

• Failed to verify username!: the player's game didn't sign in with Mojang to join. The same page covers Authentication servers are down.
• Outdated server!, Outdated client! or Incompatible client!: the game and the server run different versions.

The server lags

• Can't keep up! Is the server overloaded?: the server fell behind its 20 ticks a second, by the time and ticks it names.

The server won't start

• Error: Unable to access jarfile: Java couldn't open the file named after -jar. The same page covers Invalid or corrupt jarfile and no main manifest attribute.
• You need to agree to the EULA: a new server stops until eula.txt says eula=true.
• FAILED TO BIND TO PORT!: the server couldn't take its port, because another program has it, server-ip is wrong or the port is under 1024.

2Where the whole message is

• A player's screen shows the errors from joining: "Failed to connect to the server", then the reason.
• The server's log, logs/latest.log, has everything the server printed since it started. In Playkeeper it's the Console tab.
• Crash reports go in the server's crash-reports folder, one file for each crash, with the error's stack trace.

3What Playkeeper explains by itself

When a server stops by itself, Playkeeper reads its log and names the cause on the server's Overview, with a button for the fix where there is one. It knows these:

• The port is already in use, or the server couldn't open its port.
• The server ran out of memory, of memory for code, or hit its thread limit.
• The disk is full.
• The Minecraft EULA hasn't been accepted.
• The server needs a different Java.
• The world is already open somewhere else, its level.dat file is damaged, or part of it may be damaged.
• The server froze, or was stopped abruptly.
• A data pack stopped the world from loading.
• The server isn't allowed to use its own files.
• A mod needs another mod or a different version of one, clashes with another, or can't load.