Crash & lag troubleshooting

The console almost always tells you why. Common patterns, top to bottom:

Server stops immediately after starting

• You need to agree to the EULA (Minecraft): edit eula.txt → eula=true.
• Unsupported class file major version: your server jar needs a different Java version — change the Java/docker image in the Startup tab (e.g. MC 1.20+ wants Java 17/21).
• Missing mod dependency: Forge/Fabric prints the exact mod name it wants. Install it or remove the mod that needs it.

Server dies under load / “Out of memory”

• Console shows OutOfMemoryError or the server is killed at 100% memory: your world/modpack outgrew the plan. Options: remove heavy mods/plugins, pregenerate fewer chunks, or upgrade RAM from the client area (takes effect in minutes).

Lag (TPS drops)

• Paper: run /spark profiler or /timings report and look at the top entries — it’s nearly always a plugin or an overloaded chunk (mob farms!).
• Set sensible view-distance (6–8) in server.properties.

”Address already in use” / players can’t connect

• Make sure you’re binding the assigned port (shown in the panel), not a hardcoded one from an old config. server-ip in server.properties must stay empty.

Still stuck? Open a Technical ticket with the last 50 console lines — that’s the fastest path to a fix.