Minecraft Server Won't Start or Keeps Crashing: Causes and Fixes

First: check the console and find the first red line

1. Open the console

   On your PC, the window you started it from. On NapNodes, the server's “Console” at panel.napnodes.com: click “Start” and let everything print.

2. Look for the first error, not the last

   Scroll up to the first line with ERROR, Exception or “Caused by”. The ones after it are usually a consequence of that one.

3. If it shut down suddenly, check the files

   logs/latest.log is the console from the last run and crash-reports/ keeps the report for each crash. On NapNodes they're in “Files”.

1. “UnsupportedClassVersionError”: the Java version is wrong

The server was built for a newer Java than the one running it. Each Minecraft version needs its own: Java 8 up to 1.16, Java 17 from 1.17 to 1.20.4, Java 21 from 1.20.5 to 1.21.x, and Java 25 for the new version scheme (26.x). A newer Java only sometimes works for an old Minecraft; an older one for a new Minecraft, never. The full table and how to read the number in the error, in the Java guide.

On your PC: install the right Java and start the server with its full path. On NapNodes: if you change version or type from “Mods & plugins” → “Server type”, the panel sets the matching Java itself. If you uploaded a .jar by hand, the Java image is chosen in “Advanced settings” → “Version & startup” → “Docker Image”.

2. A mod or plugin that doesn't fit

It's the most frequent cause on modded servers. The clues in the console: “requires” or “depends on” (a dependency is missing), “Mixin apply failed” (two mods clashing or one for another version), or “net/minecraft/client” (you've put a client-only mod on the server, like a minimap or shaders). For plugins, “Could not load plugin” gives the name of the .jar.

Fix: everything for the same Minecraft version and the same loader (a Fabric mod doesn't work on Forge, nor a Paper plugin on Fabric). If you just installed something, remove it and start again; if you don't know which one it is, remove half the mods, test, and repeat with the half that fails. On NapNodes you remove them from “Mods & plugins” → “Installed”, and when installing from there only the ones that work for your version and type show up, with their dependencies.

3. “OutOfMemoryError” or it closes without a word: not enough memory

“java.lang.OutOfMemoryError: Java heap space” means there isn't enough RAM for the world, the mods or the players online. Sometimes not even that appears: the server shuts down suddenly halfway through loading, especially with big modpacks.

What to do: remove mods you don't use, lower view-distance in server.properties or pregenerate the world so it isn't generated while people play. If it still isn't enough, you need more RAM: a big modpack needs 8 GB or more. On NapNodes, if it crashes twice in an hour we email you saying whether memory was the cause, and RAM can be increased without losing anything: “More RAM” in “My plan & invoices”, or “Buy RAM” in “My servers”.

4. “FAILED TO BIND TO PORT”: the port is in use

Another program is already using that port: almost always, another copy of the same server that didn't close properly, or a server-ip in server.properties with an address the machine doesn't have. On your PC: close the other process (or restart the computer) and leave server-ip empty.

On a rented server, the host sets the address and port: don't change server-ip or server-port in server.properties, and if you did, put them back as they were.

5. The EULA or a badly written config file

“You need to agree to the EULA” appears the first time on your PC: open eula.txt and change eula=false to eula=true. On NapNodes it comes already accepted (you accept it when you buy).

If the error names a file (server.properties, a plugin's .yml or a mod's .toml), it's a badly written line: a tab in a .yml, missing quotes or a missing brace. Undo the last change you made to that file and start again.

6. The world is corrupted or from a newer version

Errors loading “level.dat” or chunks, or the world was created in a newer version than the server's. You can't downgrade on the same world: you have to go back to the version it was played on or restore an earlier backup (how to downgrade). If it starts but some areas are broken or chunks regenerate, it's a corrupted world, and it can be fixed without losing the rest.

That's why it's worth making a backup before every big change. On NapNodes, in “Backups” (with a paid plan): as well as your own, a daily backup is made automatically every night that you can restore.

If it's none of these

Copy the 20 console lines around the first error and search for the exact exception message in quotes: someone has almost always had it before. On NapNodes, open a ticket in the panel's “Help” with those lines and we'll look at it with you.

Frequently asked questions