Minecraft mod problems

This page is for modded Minecraft Java servers. Plugin servers such as Paper are covered in Minecraft plugin problems.

Forge, NeoForge, Fabric and Quilt: what is the difference?

Why it happens: They are different mod loaders. Mods are built for one loader and one Minecraft version.

How to fix it:

1. Forge is the classic loader. NeoForge is a newer project that forked from Forge and is the main choice for recent versions.
2. Fabric is light and quick to update. Quilt is a Fabric-compatible loader.
3. Mods are not interchangeable. A Forge mod will not run on Fabric.
4. Choose the software that matches your mods. See Server software and Change version.

Client and server mod mismatch (connection rejected)

Why it happens: Many mods need the same mods, and the same versions, on both the server and the player’s game. If they differ, the server refuses the connection.

How to fix it:

1. Read the error on the player’s screen. It often lists missing or mismatched mods.
2. Give players the exact same mod list and version as the server.
3. Remove client-only mods from the server (see below).
4. If you use a modpack, players should install the same version of the pack.

Wrong Minecraft version or wrong loader

Why it happens: A mod file made for 1.20.1 will not load on 1.21. Same for loaders.

How to fix it:

1. Check the Minecraft version and loader in the Console startup lines.
2. Download each mod for the exact version and loader.
3. If you must change version, do it for all mods together (see Change version).
4. Back up first (see Backups).

Missing mod dependency (Fabric API, Architectury and others)

Mod X requires mod fabric-api, which is missing

Why it happens: Many mods need a library mod. Common ones: Fabric API, Architectury API, Cloth Config, GeckoLib, Kotlin libraries.

How to fix it:

1. Read the crash or console message for the name of the missing mod and version.
2. Download it for the same Minecraft version and loader.
3. Upload it to the mods folder using Files.
4. Restart the server.

Duplicate mods

Why it happens: Two copies of the same mod, sometimes different versions, cause a crash on startup.

How to fix it:

1. Open the mods folder in Files.
2. Look for files with the same mod name and different versions.
3. Delete the older one.
4. Restart.

Client-only mods on a server

Why it happens: Some mods only work on a player’s computer, such as minimaps, shaders, or performance mods for graphics. On a server they crash or do nothing.

How to fix it:

1. Check the mod page. It says Client, Server, or Both.
2. Remove client-only mods from the server mods folder.
3. Keep them on players’ computers if they want them.

Server crashes on startup with a mod

Why it happens: A mod is broken, incompatible, or missing a dependency.

How to fix it:

1. Open Files and look for a crash-reports folder. Open the newest file.
2. Look near the top for a “Description” and the mod name. That points to the problem.
3. Also check logs/latest.log in Files, or use the Console.
4. Remove or update the mod named in the error.
5. See also Startup and crash errors and Server will not start.

Mixin errors

Mixin apply failed ... InvalidInjectionException

Why it happens: Mods use Mixin to change game code. If two mods change the same thing, or a mod is for the wrong version, it fails.

How to fix it:

1. Find the mod names in the crash report.
2. Update them to the newest version for your Minecraft version.
3. Remove one of the mods to see which two clash.
4. Report the crash log to the mod author.

Ticking entity or Ticking block entity crash

Exception ticking world / Ticking entity

Why it happens: A single entity or block, often from a mod, throws an error each tick. It is often a damaged one, or from a removed or updated mod.

How to fix it:

1. Open the newest crash report in crash-reports in Files. It lists the entity type and its coordinates.
2. Restore a backup from before it started (see Backups).
3. Update the mod that owns that entity.
4. If you know the location, a mod or editor tool can remove the entity. If not, open a ticket with the crash report.

World made with a different mod set

Why it happens: If a world was saved with mods that are now missing, their blocks and items can vanish or crash the server.

How to fix it:

1. Put back the exact mod set the world used.
2. Do not remove mods from an existing world unless you know what will disappear.
3. Make a backup before you test.

Mod configs and defaultconfigs

Why it happens: Mods store settings in the config folder. Some packs also use defaultconfigs to give new worlds a starting config.

How to fix it:

1. Stop the server before editing.
2. Open Files and go to config.
3. Edit the mod’s file, save, and start the server.
4. Per-world settings can be inside the world folder, in a serverconfig folder. Changing defaultconfigs only affects new worlds.

Adding a modpack

Why it happens: Modpacks bundle mods, configs and sometimes scripts for a matching client version.

How to fix it:

1. Download the server pack from the modpack’s page. It is a separate download from the client pack.
2. Stop the server.
3. Upload the files with Files or SFTP (see Files and Files and SFTP problems). Large packs are better sent by SFTP because the upload limit is 100 MB by default.
4. Make sure your server software matches the pack (for example NeoForge or Fabric) and the Minecraft version. Follow the pack’s own server instructions.
5. Start the server and read the Console for errors.
6. If you are stuck, open a ticket with the modpack name and version.

Which Java version does my modded server need?

Why it happens: Old Minecraft versions need older Java. Newer ones need newer Java. The wrong one gives an error like “class file version” or “unsupported major”.

How to fix it:

1. Open Startup in the panel.
2. Use the Docker Image dropdown to pick a Java version that matches your Minecraft version. As a rough guide, 1.12 to 1.16.5 use Java 8 or newer, 1.17 uses Java 16, 1.18 to 1.20.4 use Java 17, 1.20.5 to 1.21.11 use Java 21, and 26.1 and newer use Java 25. Some old modpacks are picky and need an exact Java version, so follow the pack’s own instructions. Not every Java option may be offered in the dropdown, so open a ticket if the one you need is missing.
3. Save and restart.
4. See Startup.

How much memory does a modded server need?

Why it happens: Mods add blocks, entities and recipes, and all of that uses memory. Large packs need much more than a plain server.

How to fix it:

1. Read the pack’s or the mod’s own recommended memory.
2. Compare it with your plan’s memory in Services on the client area (Provisioning Limits).
3. If you run out, see Out of memory.
4. Upgrade if the pack needs more than your plan has (see Upgrade).

Can I use plugins and mods together?

Why it happens: Plugins run on Bukkit-based software (Paper, Spigot) and mods run on loaders (Forge, NeoForge, Fabric). They use different systems.

How to fix it:

1. Choose one or the other for your server.
2. Hybrid software exists but is often unstable. We do not recommend it and support is limited.
3. For extra server-side features on Fabric or Forge, look for a mod version of what you need.

How to remove a mod safely

Why it happens: Removing a mod can delete its blocks and items from the world.

How to fix it:

1. Take a backup (see Backups).
2. Stop the server.
3. In Files, go to mods and remove the file. Also remove any mod that needs it.
4. Optionally delete the mod’s file in config.
5. Start the server and read the Console.
6. If you see errors about missing blocks or entities, restore your backup and put the mod back.

A mod update broke the world or the server

Why it happens: New versions can change how data is stored, or require newer versions of other mods.

How to fix it:

1. Restore the backup from before the update.
2. Put back the old mod version.
3. Read the mod changelog before updating next time.
4. Update mods together, and test on a copy first if you can.

Server is lagging or crashing with many mods

Why it happens: Mods add load. Heavy ones like big tech or magic mods stress the server.

How to fix it:

1. See Minecraft server lag and performance.
2. Some mod loaders have profilers. Look at their docs.
3. Remove mods you do not need.

Still stuck?

Open a ticket or ask on Discord. See Contact support. Attach the newest crash report and list your mods, loader and Minecraft version.