Fix mod and plugin errors on Minecraft

When a plugin or mod cannot load, the server either starts without it or stops during boot. When you open the Installed list, the panel reads the server log of the last start and flags the add-ons it found trouble with. This guide shows where to look, what the common messages mean and how to get the server running again.

Find the add-on that failed

1. Open the Console tab and scroll up to the first error after the last Start. Click Download Log and choose a range such as Last hour to get the recent log as a file.
2. Open the Plugins tab on Paper or Purpur, or the Mods tab on Fabric, Forge and NeoForge, and click Installed. When the Installed list opens, the panel reads logs/latest.log, the log of the last start, and marks the add-ons it found trouble with. Reopen the tab after a restart to refresh it. Failed to load means it could not load or enable. Needs followed by a name means a required add-on is missing.
3. If you added several add-ons at once, start with the newest.

What the common messages mean

On Paper and Purpur:

• Could not load ' at the start of the message: the server could not read that jar as a plugin. The lines under it give the reason.
• Unknown/missing dependency plugins: [Name]: the plugin needs another plugin first. Install the plugin in the brackets.
• Unsupported API version: the plugin was built for a newer Minecraft version than your server runs.
• Jar does not contain plugin.yml or Invalid plugin.yml: the server could not read a plugin description from that jar. Check that you uploaded a plugin built for Paper, and upload it again if the file may be damaged.
• Error occurred while enabling with (Is it up to date?): the plugin loaded and then failed while starting. Update it or disable it.
• Ambiguous plugin name: two jars of the same plugin are in the plugins folder. Delete the older one.

On Fabric, a mod that needs another mod gives a line saying it requires something which is missing!, and the summary reads Incompatible mods found!. On NeoForge the log shows Missing or unsupported mandatory dependencies: followed by the mods it wants. Install the mods named.

A Java error called UnsupportedClassVersionError means the add-on was built for a newer Java than your server runs. The panel picks Java from the Minecraft version and you cannot set it: Java 21 from 1.20.5, Java 17 for 1.17 to 1.20.4, Java 8 up to 1.16, and Java 25 from 26.x. Use a build of the add-on for your Minecraft version, or move the server to a version that runs the newer Java.

Fix it

1. Click Create backup in the Backups tab.
2. In Installed, click Disable on the add-on that failed, then Start the server. If it boots, that add-on was the cause.
3. Click Check for updates, or Update all if the button shows that instead. A newer build for your Minecraft version may fix it, and Update swaps it in.
4. If a Needs badge or a dependency message named a missing add-on, open Store and install it. When the add-on lists what it needs, the panel offers Install all.
5. Check the loader. A mod only loads on the loader it was built for, and the server may not report an error when it skips one: our test of Fabric mods on Forge found the server started with no message at all. The Mods tab starts with its Loader filter on your server's loader, so search there rather than uploading by hand.
6. If the Installed list will not load, delete the one bad jar from plugins or mods in the File Manager.
7. Still failing? Restore your backup from the Backups tab.

Fix the PluginClassLoader line covers a harmless startup line that can look like the cause. If the server will not start at all, see Find and read a Minecraft crash report.