Direct answer

A dedicated NeoForge server does not provide Minecraft client-only functionality. If the stack trace reaches client classes, identify the mod that referenced them, then update/remove that mod or use a build that correctly separates client and server code.

Diagnosis to use: Dedicated servers cannot load Minecraft client-only rendering or GUI classes during server initialization.

What it means

NeoForge dedicated servers cannot access Minecraft client-only classes. A mod that reaches them on the server side can crash.

What to collect: Inspect the first NoClassDefFoundError for packages referring to client or renderer classes, and trace the stack back to the mod loading it. Confirm the mod’s server-side support in its official metadata.

If you are a player

  • Use the exact modpack profile the server expects: Minecraft version, loader, pack release, and required client-side mods.
  • Do not “update everything” one mod at a time unless the pack author documents that combination; it can create a different mismatch.
  • Save your client latest.log or crash report from the failed attempt and give the owner the first loader/registry/Mixin error.

If you run the server

  1. Use the loader’s first dependency/sidedness error to identify the exact mod and required version range/environment.
  2. Match the intended Minecraft version, loader version, and modpack manifest rather than simply installing the newest release of each component.
  3. Keep true client-only mods off dedicated servers and ensure required server/common dependencies are present.
  4. Change one mod/dependency at a time on a test copy and keep the original pack/world backup for rollback.

Targeted repair: Remove or replace the client-only mod in the staging server’s mods directory. Keep the matching client-side component on player computers if the pack requires it there.

Verify after the change: Start the dedicated server headlessly; verify no client class appears in the startup trace and that clients can connect.

References

Use the documentation for the exact software and release you run. These sources support the settings or diagnostic behavior used in this fix.

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