Common Minecraft Server Errors and How to Fix Them

Startup Errors

"Error: Could not find or load main class"

Cause: Java can't find the server JAR file.

Solution:

1. Make sure the JAR file is in the same directory as your start script
2. Check the JAR filename matches exactly what's in your start script
3. Remove spaces from folder names in the path

# Correct
java -jar server.jar nogui

# Wrong - file doesn't exist
java -jar Server.jar nogui

"UnsupportedClassVersionError" or "class file version 65.0"

Cause: Wrong Java version. Minecraft requires specific Java versions:

Solution:

1. Install the correct Java version from Adoptium
2. Update your start script to use the new Java

# Point to specific Java installation if needed
/path/to/java21/bin/java -jar server.jar nogui

"Failed to bind to port" or "Address already in use"

Cause: Another process is using port 25565.

Solution:

1. Check for other Minecraft servers running
2. Close any conflicting applications
3. Use a different port:

# In server.properties
server-port=25566

Need a step-by-step walkthrough? See Fix Failed to Bind to Port.

"EULA not accepted"

Cause: You haven't agreed to the Minecraft EULA.

Solution: Open eula.txt and change eula=false to eula=true.

Connection Errors

"Can't connect to server" or "Connection refused"

Common causes and solutions:

1. Wrong IP address

   • Use localhost for same computer
   • Use your local IP (192.168.x.x) for LAN
   • Use your public IP for external connections

2. Firewall blocking

   # Windows - allow through firewall
   netsh advfirewall firewall add rule name="Minecraft" dir=in action=allow protocol=TCP localport=25565
   

3. Port not forwarded

   • Access your router admin panel
   • Forward port 25565 (TCP) to your server's local IP

"io.netty.channel.AbstractChannel$AnnotatedConnectException"

Cause: Network connection failed.

Solutions:

• Check the server is actually running
• Verify the IP address and port are correct
• Check for firewall/antivirus blocking
• Try restarting your router

If you see "Connection timed out", follow Fix Connection Timed Out.

"Outdated server" or "Outdated client"

Cause: Minecraft version mismatch between client and server.

Solution:

• Update your client to match the server version, OR
• Update your server to match your client version

Crashes and Errors

"OutOfMemoryError: Java heap space"

Cause: Not enough RAM allocated.

Solution: Increase memory in your start script:

# Increase from 4G to 6G
java -Xms6G -Xmx6G -jar server.jar nogui

"Ticking entity" crash

Cause: A specific entity is causing the server to crash.

Solutions:

1. Check the crash report for coordinates
2. Use MCEdit or NBTExplorer to remove the entity
3. Restore from backup if available

"Chunk loading errors" or corrupted world

Cause: World files are damaged.

Solutions:

1. Restore from backup (best option)
2. Use region fixer tools
3. Delete the corrupted region files (causes data loss)

Plugin conflicts and errors

Cause: Incompatible or buggy plugins.

Solution:

1. Check logs/latest.log for error messages
2. Remove plugins one by one to identify the culprit
3. Update plugins to latest versions
4. Check plugin compatibility with your server version

Performance Issues

Low TPS (below 20)

Cause: Server is lagging and can't keep up.

Solutions:

1. Use Spark profiler to identify issues
2. Reduce view-distance in server.properties
3. Remove performance-heavy plugins
4. Pre-generate your world

See our Optimization Guide for detailed solutions.

High memory usage

Cause: Memory leak or too many loaded chunks.

Solutions:

1. Set a world border to limit exploration
2. Use Paper's chunk unloading features
3. Check for plugin memory leaks with Spark
4. Restart server on a schedule

Tick lag spikes

Cause: Something causing periodic freezes.

Common culprits:

• Auto-save (every 5 minutes by default)
• Backup plugins
• Large redstone contraptions
• Entity processing

Error Logs

Where to find logs

Reading crash reports

Key sections to check:

1. Description - What happened
2. Stack trace - The technical error
3. System Details - Java version, OS, etc.
4. Mods/Plugins - List of loaded additions

Prevention Tips

1. Keep backups - Backup world and config files regularly
2. Test changes - Test new plugins on a development server first
3. Update carefully - Read changelogs before updating
4. Monitor resources - Use monitoring tools to catch issues early
5. Use stable software - Prefer stable releases over development builds

Related Guides

• Server Optimization Guide
• How to Install Paper
• Startup Flags Generator