Common Minecraft Server Errors
Server won't start? Here are the most common reasons and how to fix them.
If your error is not on this list, the log will name it. See Minecraft keeps crashing: find the log and read it for where latest.log lives and the three lines in it worth reading.
1. "You need to agree to the EULA"
Error: The server stops immediately after downloading files.
Fix: Open the eula.txt file and change eula=false to eula=true. Save and restart.
2. "Java version mismatch" / "Unsupported Class Version"
Error: Exception in thread "main" java.lang.UnsupportedClassVersionError
Cause: You are using an old version of Java for a modern Minecraft version.
Fix:
- Minecraft 26.1 and newer needs Java 25. It will not start on Java 21 and says so in the log.
- Minecraft 1.20.5 to 1.21.11 needs Java 21.
- Minecraft 1.18-1.20 needs Java 17.
- Minecraft 1.16 needs Java 8 or 11.
3. "Failed to bind to port"
Error: FAILED TO BIND TO PORT!
Cause: Another server is already running on port 25565, or you are trying to use a port that is already taken.
Fix:
- Check if a "ghost" server process is running in the background and kill it.
- Change the `server-port` in `server.properties` to something else (e.g., 25566).
4. "Watchdog" Crash
Error: The server has stopped responding! (Watchdog)
Cause: The server is frozen or lagging so badly it shut itself down.
Fix: Increase the max-tick-time in server.properties to -1 (to disable it) or 60000. Also, check your CPU/RAM usage.
5. Modded server crash: NoClassDefFoundError: net/minecraft/client/...
Error: a modded (NeoForge/Fabric) server crashes on startup referencing a net.minecraft.client class, or "invalid dist DEDICATED_SERVER".
Cause: a client-only mod is in the server's mods/ folder. A dedicated server has no graphics classes, so client-side code crashes it.
Fix: identify and remove the client-only mod. See the full walkthrough in Modded Server Won't Start: Client-Only Class Crashes.
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