How to Fix a Minecraft Server That Won't Start

Diagnose and fix common reasons your Minecraft Java server fails to start — EULA, Java version, RAM, corrupted JAR, mods, world corruption, exit codes.

When a Minecraft server refuses to start, the first error in the Console almost always tells you exactly what's wrong. Most startup failures fall into one of seven categories: EULA, Java version, RAM, mods/plugins, corrupted JAR, world corruption, or port conflict.

The golden rule of Minecraft troubleshooting: Read the first error in the Console, not the last. Later errors are usually side effects of the first.

Diagnostic Flow

Open Console and read the first error

In the XGamingServer Panel, open Console. Scroll to the start of the latest startup attempt. Look for the first line containing ERROR, Exception, FATAL, or Failed.

Match the error to the table below

Use this table to identify the cause and jump to the fix.

Errors and Fixes

`You need to agree to the EULA`

Minecraft requires you to accept Mojang's EULA before the server runs.

Fix: Open Files, edit eula.txt, change eula=false to eula=true, save, and restart. See Accept the EULA.

`UnsupportedClassVersionError` or `has been compiled by a more recent version of the Java Runtime`

Your server JAR was built for a newer Java than the one running it.

Minecraft version	Required Java
1.20.5 and newer	Java 21+
1.18 – 1.20.4	Java 17+
1.17	Java 16+
1.13 – 1.16	Java 8 or 11
1.12 and older	Java 8

Fix: Open Startup in the panel and select the matching Java version (or Docker image). See Java Version.

`OutOfMemoryError` / `Java heap space` / Exit code 137

The server ran out of allocated RAM.

Fix: Open Startup and increase memory. See How Much RAM. Also check for memory leaks: a 4GB server hitting OOM with 5 players probably has a runaway entity or chunk loader. See Optimize Server.

`Address already in use`

Another process is using the port.

Fix: Rare on hosted servers. Restart the server from the Dashboard. If it persists, contact support — the previous process may not have released the port.

Crash on plugin or mod load (`NoClassDefFoundError`, `ClassCastException`, `NoSuchMethodError`)

A plugin or mod is incompatible with your Minecraft version, your server software, or another mod.

Fix: The error trace usually names the offending plugin/mod. Remove the most recently added file from plugins/ or mods/ and try starting again. See Find / Read Crash Reports.

`Session lock` / `level.dat is missing` / `chunk loading failed`

World file corruption — usually from an unclean shutdown.

Fix options (in order):

Delete world/session.lock (safe — it's a lock file, gets recreated)
Restore from Backups in the panel
Replace level.dat with level.dat_old from the world folder
Use an offline world editor (NBTExplorer) to inspect

See Fix: Session Lock.

Exit code	Meaning
`0`	Clean shutdown
`1`	General error — read the Console
`137`	Out of memory (OOM kill) — increase RAM
`139`	Segfault — usually JNI/native lib bug
`143`	Server received SIGTERM (manual stop or timeout)

Full reference: Java Exit Codes.

Diagnostic Checklist

Run through this in order — each step rules out one common cause:

✅ EULA accepted? (eula.txt → eula=true)
✅ Java version matches Minecraft version? (Startup tab)
✅ RAM sufficient? (no exit 137 in history)
✅ No new mods/plugins added since last working start? (remove and retry)
✅ JAR not corrupted? (Reinstall Server)
✅ World not corrupted? (delete session.lock, restore backup)
✅ Console first error read? (not just the last line)

If all 7 pass and the server still won't start, open a support ticket with the full Console log from the failed attempt.

Common Mistakes

Mistake	Fix
Reading only the last error	Read the first error — later ones are side effects
Reinstalling the server (loses configs)	Reinstall is safe — keeps world and configs by default
Deleting `world/` to "fix" startup	Almost never the problem — and you lose everything
Ignoring the Java version warning	Wrong Java is the #2 cause of startup failures
Restarting in a loop without reading Console	Each crash leaves the same error — read it once