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UnsupportedClassVersionError means your Minecraft server, or a plugin or mod, was built for a newer Java version than the one your server runs. Fix it on FREAKHOSTING by opening Startup, choosing a newer Java under Docker Image (Java 25 for Minecraft 26.1 and newer, Java 21 for 1.20.5 to 1.21.11) and restarting the server.

How to read the error

The error looks like this:
The first number is the Java the file needs. The second is the Java your server has. In the example, the file needs Java 25 (69.0) and the server runs Java 21 (65.0).

Which Java does my Minecraft version need?

Check your version in Version Changer (status line Currently installed: …).

How to fix it

1

Open Startup

Click Startup in the sidebar.
2

Pick a newer Java

Under Docker Image, choose the Java version from the tables above. The choices are Java 25, 21, 18, 17, 16, 11 and 8.
Startup page in the FREAKHOSTING Game Panel with the Docker Image drop-down listing Java versions from 25 to 8

Choose the Java version under Docker Image

3

Restart

Restart the server and watch the console. The error should be gone.
Version Changer picks the right Java for you when you install a version. You usually see this error after uploading your own server jar, or after installing a plugin or mod made for a newer Minecraft version.

What if a plugin or mod causes it?

If the error names a plugin or mod rather than the server, that file is made for a newer Minecraft (and Java) than your server. Don’t just raise Java: the plugin or mod probably won’t work on your Minecraft version either. Instead:
  1. Open Plugins or Mods > Installed and delete it, or switch it off.
  2. On Browse, install a version listed under Versions for this server.
  3. Restart.

Frequently asked questions

What does “class file version 65.0” mean?

It means Java 21. A file with class file version 65.0 needs Java 21 or newer to run.

Can I always use the newest Java?

Newer Minecraft versions need newer Java, but very old versions and some old modpacks only start on Java 8. Use the version in the table.

I changed Java and now the server won’t start. What now?

Set Docker Image back to the previous Java and restart. Then check the error in Logs: it may be a different problem. See server crashing.

Does UnsupportedClassVersionError affect players?

No. It’s a server error. Players can’t join until the server starts, but nothing on their side needs changing.

Minecraft server guide

Every Minecraft guide in one place.

Startup flags and Java

The Startup page explained.

Change your server version

Version Changer sets Java for you.

Server crashing

Other start-up errors.
Tested by the FREAKHOSTING team on our Game Panel on 29 October 2026 (Minecraft 26.1, Paper and Fabric). Java requirements checked against the official Minecraft 26.1 release notes.

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Last Updated: October 2026