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Minecraft Server Crashing? Common Errors and How to Fix Them

What “Failed to bind to port”, “Could not reserve enough space for object heap”, OutOfMemoryError, UnsupportedClassVersionError and other Minecraft server errors mean, and how to fix each one.

When a Minecraft server stops, the reason is almost always in the last 20 lines of the console. Find the message below that matches yours. VoxelPort recognises most of these automatically and shows the explanation and a fix button when your server stops.

“FAILED TO BIND TO PORT” / “Address already in use”

Meaning: another program is already using the server’s port, usually another Minecraft server that’s still running.

Fix: close the other server (check Task Manager for java.exe), or change server-port in server.properties (in VoxelPort: the server’s Settings).

“Could not reserve enough space for object heap”

Meaning: the server is set to use more RAM than Java can get. It’s common with 32-bit Java or when the RAM setting is higher than the free memory on your PC.

Fix: lower the RAM (the -Xmx value), and make sure you use 64-bit Java.

“java.lang.OutOfMemoryError”

Meaning: the server ran out of the RAM it was given, from too many players, chunks or mods.

Fix: give it more RAM, lower view-distance and simulation-distance, or remove heavy mods.

“UnsupportedClassVersionError … class file version 69.0”

Meaning: the server needs a newer Java than the one running it. Class file version 61 means Java 17, 65 means Java 21, and 69 means Java 25.

Fix: install the right Java (Java 25 for Minecraft 26.1+, Java 21 for 1.20.5–1.21.x) and start the server with it. VoxelPort picks and installs the right Java automatically.

“You need to agree to the EULA in order to run the server”

Meaning: every server must accept Mojang’s EULA once.

Fix: open eula.txt in the server folder and change eula=false to eula=true, after reading the EULA at minecraft.net/eula.

Mod errors: wrong version, missing dependency, Mixin failures

“Could not load 'plugins/…'” / “Error occurred while enabling”

Meaning: a Paper/Spigot plugin failed, usually because it’s built for a different Minecraft version.

Fix: update the plugin, or move it out of the plugins folder to confirm it’s the cause.

“Invalid or corrupt jarfile” / “Unable to access jarfile”

Meaning: server.jar is missing, renamed or only partly downloaded.

Fix: download the server jar again and make sure the start command uses the right file name.

“session.lock” / “The directory is already locked”

Meaning: another Minecraft process has the world open.

Fix: close the other server or game using that world (end java.exe in Task Manager), then start again.

“AccessDeniedException” / “Access is denied”

Meaning: the server can’t write to its folder. It happens with read-only folders and folders synced by OneDrive.

Fix: move the server to a normal folder such as Documents (outside OneDrive), and don’t run it from a ZIP file or a protected location like Program Files.

The server doesn’t crash but lags

Check the server’s TPS (ticks per second). 20 is perfect; below about 15 players will notice. Lower the view distance, limit farms with many entities, and use Paper instead of Vanilla for better performance. VoxelPort shows CPU, RAM and TPS live while the server runs.

Questions

Where do I find the crash reason?

Look at the last lines of the server console or the logs/latest.log file. Mod and plugin crashes also write a file in the crash-reports folder. VoxelPort reads the console for you and explains common causes in plain English.

How much RAM should a Minecraft server have?

About 2–4 GB for a small vanilla or Paper server, and 6–10 GB for big modpacks. Leave at least 2–3 GB for Windows and anything else running on the PC.

Which Java version does my server need?

Minecraft 26.1 and newer need Java 25. Minecraft 1.20.5 to 1.21.x need Java 21, and 1.18 to 1.20.4 need Java 17.

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