UnsupportedClassVersionError class file version Minecraft

What this actually means

The server (or a mod) was compiled for a newer Java than the one running it. Class file version 52=Java 8, 60=Java 16, 61=Java 17, 65=Java 21.

How to fix it

Minecraft 1.17-1.20.4 wants Java 17. 1.20.5+ and 1.21 want Java 21. 1.16 and older want Java 8. Point your start script at the right JDK explicitly rather than relying on whatever `java` resolves to on PATH -- that is usually the cause.

What it looks like in your log

Search your crash report or latest.log for any of these:

UnsupportedClassVersionError  ...  class file version
has been compiled by a more recent version of the Java Runtime
java.lang.UnsupportedClassVersionError

The free edition of Forge Server Doctor detects this one. It is a single Python file with no dependencies — point it at the report and it names the cause and the mod jar:

python3 forge_doctor.py crash-reports/crash-....txt