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"UnsupportedClassVersionError"

The server is built for a newer Java than the one it's started with. Install the Java version it needs, or run the server from ProximaClient: the app installs the Java that fits the chosen Minecraft version itself.

In the server console it looks like this:

Error: A JNI error has occurred, please check your installation and try again
java.lang.UnsupportedClassVersionError: net/minecraft/server/Main has been compiled by a
more recent version of the Java Runtime (class file version 65.0), this version of the
Java Runtime only recognizes class file versions up to 61.0

The first number is the Java the server needs, the second is the Java it was started with: 52 is Java 8, 60 is Java 16, 61 is Java 17, 65 is Java 21, 69 is Java 25.

What to do​

If the server runs from ProximaClient, open the "Launch options" tab, set "Java version" to "Auto" and restart the server. If you run the server yourself:

  1. Find out which Java your Minecraft version needs:

    Minecraft versionJava
    1.16.5 and earlier8
    1.1716
    1.18 – 1.20.417
    1.20.5 – 1.21.x21
    26.1 and newer25

  2. Install that version, for example Eclipse Temurin.

  3. Check the version with java -version in the Command Prompt and restart the server.

If there are several Java versions on the system, the server may start with the wrong one: put the path to the right java.exe in the server's start script.

If that didn't help​

Write to support through "Tech support" in the app menu with the error text from the console and when it appeared.