Direct answer
Remove a truly client-only Fabric mod from the dedicated server. Fabric’s `environment: client` means that mod is not loaded on dedicated servers; if a mod that should support servers crashes on client-only classes, use a server-compatible build or report the sidedness bug to its author.
What it means
Dedicated servers do not provide client-only Minecraft classes, so a mod that wrongly loads client code can crash the server.
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.logor crash report from the failed attempt and give the owner the first loader/registry/Mixin error.
If you run the server
- Use the loader’s first dependency/sidedness error to identify the exact mod and required version range/environment.
- Match the intended Minecraft version, loader version, and modpack manifest rather than simply installing the newest release of each component.
- Keep true client-only mods off dedicated servers and ensure required server/common dependencies are present.
- Change one mod/dependency at a time on a test copy and keep the original pack/world backup for rollback.
References
Use the documentation for the exact software and release you run. These sources support the settings or diagnostic behavior used in this fix.
Focused diagnostic check: Find the first client-only class in latest.log (for example a reference to client rendering or MinecraftClient), then check which mod JAR owns that class. Remove only confirmed client-only artifacts from a COPY of the server mods folder; keep shared libraries that server-side mods actually require.
Related fixes and guides
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