Minecraft Server Mod Errors: When Joining and When Starting

First: does it fail when joining or when starting?

If the server is on and someone gets a message when connecting, the problem is in that player's game: a mod is missing, they have another version or they opened Minecraft without the loader. If others join fine, the server is healthy.

If the server never finishes starting or shuts down halfway through loading, the problem is on the server. The cause shows up in the panel's “Console” when you press “Start”, and if it closed suddenly, in logs/latest.log and in the crash-reports folder (in “Files”).

When joining: «Mod rejections» or «missing mods»

On Forge and NeoForge, the server compares its mods with the player's when they connect. If they don't match, a list appears with «Mod rejections», «Mismatched mod channel list» or the missing mods, and sometimes the version it expects for each one. It's the most common one: someone with the modpack from two weeks ago, or who added a mod on their own.

Fix: everyone with exactly the same mods and versions as the server. The easiest way is not to hand them out manually. On NapNodes, in the server's “Mods & plugins”, the “For your players” section has a link (napnodes.com/jugar/…) with “Copy the link”: whoever opens it imports into Prism Launcher the modpack with the server's mods, the loader and the version, and joins. It updates itself when you change the server's mods, so they just need to import it again.

On Fabric the check is less clear: sometimes it kicks you while loading the world with a «registry» error (blocks or items the server has and your game doesn't) or a packet it doesn't understand. Same thing: mods are missing or the versions aren't the same.

When joining: the server asks for Forge, NeoForge or Fabric

Messages like «Incompatible FML modded server», «This server requires … to be installed» or the server showing up in red in the list: the player opened normal Minecraft, or the profile for another version. They have to launch the profile with the same loader (Forge, NeoForge or Fabric) and the same Minecraft version as the server. How to join a modded server, step by step.

It also happens the other way round: with a modded profile you can join a server without mods (Paper) only if those mods are client-side (Sodium, minimaps, shaders). The ones that add new blocks or items won't let you join a server that doesn't have them.

When starting: a mod needs another one that isn't there

On Forge and NeoForge you get «Missing or unsupported mandatory dependencies» with the mod that's asking and what it's asking for; on Fabric, «Incompatible mods found!» and lines like «Mod X requires … of Y, which is missing». Many mods need a separate library (Fabric API, Architectury, Cloth Config, GeckoLib…), or a minimum version of another mod or of the loader itself.

Fix: install whatever it names, for the same Minecraft version and the same loader, or update the one that's out of date. If what it asks for is another version of Minecraft or of the loader, that mod isn't for your server: look for the version of the mod that matches. On NapNodes, whatever you install from the “Mods & plugins” search already brings its required dependencies and only shows versions that work on your server; this error usually comes with mods uploaded by hand.

When starting: there's a client-only mod

On Forge and NeoForge: «Attempted to load class net/minecraft/client/… for invalid dist DEDICATED_SERVER». On Fabric: something like «Cannot load class net/minecraft/client/… in environment type SERVER». Both say the same thing: one of those mods that only change what you see (graphics, interface, minimaps, shaders, zoom) is in the server's mods folder, and the server has no screen.

It almost always happens after copying the whole mods folder from your PC to the server. Fix: remove that mod from the server and keep it only in each player's game. Sodium, Iris or OptiFine, for example, don't go on a server. The mod's name appears a few lines below or above the error; if not, look up which mod has that class. The “Mods & plugins” search only shows mods that work on a server, so it doesn't install them.

When starting: two mods that clash («Mixin apply failed»)

«Mixin apply failed», «MixinApplyError» or «InvalidInjectionException» with a mod's name: that mod is trying to change a part of the game another one has already changed, or it's for another Minecraft version. Check which mod the line names (it's in the name of the .mixins.json file) and what you added or updated last.

Fix: update both to their latest version for your Minecraft; if they still clash, keep one. Optimization mods that do the same thing (two that rewrite lighting, for example) tend to clash with each other.

Reading a crash report

Every time the server crashes, it leaves a file with the date in crash-reports. At the very top, «Description» says what happened in one sentence. Then comes the error with its «Caused by» lines: the last «Caused by» is usually the real cause. Forge and NeoForge ones also include «Suspected Mods», which often already names the culprit.

If it isn't clear, look in the report for package names that aren't Minecraft's (whatever doesn't start with net.minecraft or java.): they usually carry the name of the mod or its author. And search for the exact error sentence in quotes: someone has almost always had it before with the same mod. Or paste the log into the crash log analyzer, which tells you what failed.

If you don't know which one: remove them by halves

1. Make a backup

   In the panel's “Backups”, so you can go back if something goes wrong (on paid plans, one is also made automatically every night). The free server has no backups: there, move the mods aside instead of deleting them.

2. Move half the mods aside

   In “Files”, create a folder outside mods (for example mods-aside) and move half of them there. Keep inside the libraries the others need.

3. Start it and check

   If it starts, the culprit is in the half you moved aside; if not, in the half that's left. Repeat with that half. With 100 mods, you'll have it in seven or eight rounds.

4. Put the good ones back

   Put all of them back except the culprit and look for another version of it or remove it. If players use the “For your players” link, they should import it again.

And if it isn't a mod

If the console says «OutOfMemoryError» or the server closes without saying anything halfway through loading, it's memory, not a broken mod: big modpacks ask for 8 GB or more (the RAM guide goes by size). If it says «UnsupportedClassVersionError», it's the Java version. Both are in My Minecraft server won't start, with the other causes.

On NapNodes, if you get stuck, open a ticket in the panel's “Help” with the error lines and we'll look at it with you.

Frequently asked questions