Why won’t my Minecraft server start? Reading the crash and fixing it

When a server fails to start, the panel shows it as Crashed or Ran out of memory rather than leaving it stuck on "Starting". The reason is always written down in the server’s log — the hard part is knowing where to look and what the message means. This guide covers both. Nearly every startup crash comes down to one of a handful of causes, and almost all of them are fixed by removing or replacing one file.

Where to find the error

• The Console tab shows the log of the server as it starts, so a crash during startup is visible there.
• `logs/latest.log` in the Files tab is the same log saved as a file. It stays there after the server stops, so you can read it at your own pace. Older runs are kept in the same folder as dated .log.gz files.
• The `crash-reports` folder in the Files tab. When Minecraft itself crashes, it writes a full report here, with a plain-language description near the top.

How to read a Java error

Minecraft crashes come with long "stack traces" — dozens of lines starting with at …. You can ignore nearly all of it. What you want is:

1. The first line containing ERROR, Exception or FATAL during startup. Errors further down are usually consequences of the first one.
2. Any line starting with `Caused by:` — the last one in a block is normally the real root cause.
3. Any mod or plugin name in those lines. A mod’s ID (for example create or jei) or a jar file name tells you who to blame.

Then match what you found against the causes below.

Ran out of memory

Messages: java.lang.OutOfMemoryError: Java heap space, or the panel status Ran out of memory.

The server needed more than the 3 GB a free server has. This is common with large modpacks, and it cannot be tuned away — a 300-mod pack simply needs more memory. Remove mods you do not use, choose a lighter pack, or move to a paid server with more memory. How much RAM does a Minecraft server need? shows what fits.

A client-only mod on the server

Message: Attempted to load class … for invalid dist DEDICATED_SERVER, often naming a class from a rendering, shader, minimap or menu mod.

Some mods only make sense on a player’s computer — they change graphics, sound or menus — and they crash a server that tries to load them. Find the mod named near the error and delete its jar from the mods folder. When you install a CurseForge modpack, Cobbly already removes known client-only mods and retries automatically when it spots this error, so you mainly hit it with mods you added by hand.

A missing dependency

Messages: Missing or unsupported mandatory dependencies, requires … which is missing, or Unknown dependency.

Many mods and plugins need another one — a library — to work. The error names both: the mod that failed and the one it needs. Install the missing one, making sure it matches your Minecraft version. The single most common case is a Fabric mod without Fabric API. On Paper, it is often Vault or ProtocolLib.

A mod or plugin for a different version

Messages: Incompatible mod set, requires minecraft …, Mod … is incompatible with loaded version, Unsupported class file major version, or has been compiled by a more recent version of the Java Runtime.

Mods are built for one exact Minecraft version and one loader. A 1.20.1 mod will not load on 1.21, and a Fabric mod will not load on Forge. Check the file you installed against the server’s version on the Software tab, and download the matching build. The Java messages mean the same thing in disguise: Cobbly runs the Java version your Minecraft version needs, so a mod asking for different Java was built for a different Minecraft version.

Two mods that conflict

Messages: Mixin apply failed, MixinApplyError, or InvalidInjectionException, usually naming two mods.

Mods patch Minecraft’s code, and occasionally two try to patch the same thing in incompatible ways. If the error names two mods, remove the one you added most recently and start again. If you just added several at once, remove half, test, and narrow it down from there. An update to either mod often fixes it.

Duplicate mods

Message: Found duplicate mods or Duplicate mod ID.

Two jars of the same mod, often two different versions after an update. Open the mods folder and delete the older one.

A broken world or config file

Messages: errors mentioning level.dat, Failed to load level, or a parse error naming a file under config/.

A config file with a typo, or a world that did not save cleanly, can stop startup. A config error names its file — undo your last change to it, or delete it so the mod writes a fresh default. For world errors, restore the latest good copy from the Backups tab (see How backups work).

The server crashes later, not at startup

Message: A single server tick took 60.00 seconds (should be max 0.05).

This is Minecraft’s watchdog shutting the server down because it froze for a full minute. Something was doing far too much work — a giant farm or redstone machine, thousands of dropped items, or a mod stuck in a loop. The crash report lists what the server was busy with. Why is my Minecraft server lagging? covers the usual culprits.

The fix: removing a file

For every cause above, the fix is the same few steps:

1. Make sure the server is stopped.
2. Open the Files tab and go to mods (or plugins on Paper and Purpur).
3. Delete, or replace with the right version, the jar the error named.
4. Press Start and watch the Console tab until you see the line starting with Done (.

If the error does not name anything you recognise, the support page lists the ways to reach us. Include the first ERROR and the last Caused by: lines — that is usually enough to point at the answer.