Why your modded Minecraft server crashes on startup

By kimolo · 11 October 2026

You export your modpack from your launcher, upload it to a server, press start, and it dies within seconds. Nine times out of ten the pack is fine. The problem is that it contains mods that only belong on the client: the game you play, not the server you join.

What it looks like

Here's a real one from a NeoForge 26.2 server running an Alex's Caves pack. The server got about a second into starting and stopped with:

Running graphics bootstrap plugin sodium
java.lang.NoClassDefFoundError: org/lwjgl/Version
  at ...sodium.client.compatibility.checks.PreLaunchChecks...

LWJGL is the library Minecraft uses to draw the game window. A dedicated server has no window, so it doesn't ship that library. Sodium, a rendering mod, went looking for it and crashed the whole server. When a log mentions graphics, rendering, shaders, the window or LWJGL, think client-only mod first.

The usual suspects

These are client-side mods that turn up in exported packs all the time. That same pack had eleven of them:

  • Performance and rendering: Sodium, Iris (shaders), ImmediatelyFast, Entity Culling
  • Visuals: Entity Model Features, Entity Texture Features, LambDynamicLights
  • Controls and quality of life: Zoomify, Mouse Tweaks, Highlighter, CutThrough

Depending on the loader and version, some of these just do nothing on a server instead of crashing it, but none of them belong there.

How to check any mod

  1. Look the mod up on Modrinth. Each project lists whether it's needed on the client, the server, or both. "Server side: unsupported" means take it off the server.
  2. Don't trust the pack's own labels. A .mrpack can mark each file as client-only, but packs exported from a launcher often mark every mod as needed everywhere.
  3. Read the first error in the log, not the last. It usually names the mod jar that caused it.

The fix

Stop the server and move the client-only jars out of the server's mods folder into a folder of their own. Don't delete them: your players still need them in their own game. Start the server again.

My server panel now does this automatically. After a modpack installs, it checks every mod against Modrinth and sets aside anything marked client-only before the first start.

If it still won't start

  • Wrong Java version. Minecraft 1.20.5 to 1.21.x needs Java 21. The 26.x versions need Java 25.
  • A mod for a different version or loader. A Fabric mod in a NeoForge pack, or a 1.21 mod on 26.2. The file name usually gives it away.
  • A missing dependency. The log names the mod it wants and the version range. Install that and try again.
  • Not enough memory. Big packs want 6 to 8 GB. "OutOfMemoryError" in the log means give it more.

I set up and look after modded Minecraft servers. If yours won't start, send me the log and I'll tell you what's wrong.

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