Why won't my Minecraft server start?

Open your server in the panel and go to Minecraft → Overview. It reads the latest log and crash report and looks for the usual causes:

• Wrong Java version. Each Minecraft version needs a particular Java. There's a one-click fix.
• The EULA hasn't been accepted. Also a one-click fix.
• A client-only mod on the server, such as a minimap or shader mod.
• A missing dependency. A mod needs another mod you haven't installed.
• A mixin conflict. Two mods are changing the same part of the game.
• Out of memory. Too many mods for the server's RAM.
• A world downgrade. The world was saved by a newer version than the server runs.
• The watchdog. The server froze for too long and shut itself down.
• Port problems. Usually from changing server-port by hand.

It also shows what the server is really running: the Minecraft version, the loader and its version.

Still stuck?

Read the console from the top of the last start. The first error is usually the real one, and a broken mod normally names itself. Remove the most recent change and try again.