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Tickwise
CriticalStartupSIG-ST-001

Java too old for the server or a plugin (UnsupportedClassVersionError)

Platforms
Paper · Purpur · single server
Verified
Verified
Confidence
Confidence 97%

What it looks like in the log

Any one of these lines is enough. The agent matches them locally, on your server.

logs/latest.log
  • has been compiled by a more recent version of the Java Runtime

Symptoms

  • The server fails to start right after the JVM launches, or a single plugin fails to load
  • In the log: “has been compiled by a more recent version of the Java Runtime (class file version N)”

Cause

A class (the server core or a plugin) was compiled for a newer Java than the one the server runs on. The class file version number indicates the required Java: 65 = Java 21, 69 = Java 25.

Possible causes

  • The server/panel image runs on an old Java (e.g. 17), while Paper 1.21.x requires Java 21 and Paper 26.x requires Java 25
  • The plugin was compiled for Java 25 while the server runs on Java 21 (a plugin for a newer Minecraft version)
  • The wrong Docker image/java path is selected in the panel

How to fixMedium risk

  1. Find two numbers in the evidence: class file version X (required) and up to Y (currently available). 65.0 = Java 21, 69.0 = Java 25, 61.0 = Java 17.
  2. If the line mentions a core class (org/bukkit/craftbukkit/Main, io/papermc/...), the server will not start: install the required Java (Paper 1.21.x — Java 21, Paper 26.x — Java 25) and change the Docker image/Java version in the hosting panel.
  3. If it mentions a plugin class, either upgrade the server's Java or get a plugin build made for your Java/Minecraft version.
  4. Verify with java -version using the same java path the server is launched with.

Translated from the Russian original; log lines are quoted verbatim.

Sources