Minecraft server errors and how to fix them

When a Minecraft server will not start or stops, the answer is almost always in the console or in logs/latest.log. Scroll to the first error, not the last one: later messages are usually the result of the first. Here are the messages people meet most often and what to do about each.

You need to agree to the EULA

You need to agree to the EULA in order to run the server. Go to eula.txt for more info.

Open eula.txt and change eula=false to eula=true. By doing so you accept Mojang's EULA. On Saryno the create form asks you to accept it.

Address already in use

java.net.BindException: Address already in use
**** FAILED TO BIND TO PORT!

Something else is already using the server's port (25565 by default). Usually it is another copy of the server still running. Stop it, or change server-port in server.properties.

UnsupportedClassVersionError

java.lang.UnsupportedClassVersionError: ... has been compiled by a more recent version of the Java Runtime (class file version 65.0), this version of the Java Runtime only recognizes class file versions up to 61.0

The server needs a newer Java than the one that started it. Class file version 61 is Java 17, 65 is Java 21 and 69 is Java 25. Install the right one, and check that your start command uses it. The Java version checker shows which Java each Minecraft version needs.

OutOfMemoryError

java.lang.OutOfMemoryError: Java heap space

The server ran out of memory. Give it more with -Xmx, lower view-distance, or remove memory-hungry plugins. The RAM calculator suggests a size.

Can't keep up!

Can't keep up! Is the server overloaded? Running 2345ms or 46 ticks behind

A tick took far longer than the 50 milliseconds it should take. This is lag, not a crash, and it has many causes. Work through how to fix server lag: lower the distances, pre-generate the world and profile with spark.

The server has stopped responding

The server has stopped responding! This is (probably) not a Paper bug.

That is Paper's watchdog. It thinks the main thread has been stuck for longer than timeout-time (60 seconds by default) and prints a thread dump. A thread dump shows what the server was doing, so look for a plugin name in the "Server thread" section. A plugin doing slow work, a huge redstone machine or a very overloaded server can trigger it. It can also happen if the machine's clock jumps, which we wrote about in how Saryno puts servers to sleep.

Could not load plugin

Could not load 'plugins/Example.jar' in folder 'plugins'
Unsupported API version 1.21

The plugin was built for a different Minecraft version than your server. Download the right version, or wait for an update. Missing dependencies and a Java mismatch show similar messages, see how to install plugins.

Outdated server / outdated client

A player who tries to join with a different version sees "Outdated server!" or "Outdated client!". Either they change their version, or you add ViaVersion to let other versions in.

Connection refused or timed out

The player cannot reach the server at all. Check that the server is running, that the address and port are right, and (on your own computer) that the router forwards the port. See hosting at home versus hosted.

Failed to verify username

Failed to verify username!

Mojang's login service could not confirm the player. Restarting the game or the launcher often fixes it, and so does waiting a few minutes if Mojang's servers are busy. Do not turn online-mode off to fix it, see how to protect your server.

A good habit when it breaks

Back up before you change anything.
Read the first error in the log.
Change one thing at a time.
If you ask for help, paste the log, not a screenshot of a single line.