"Restricted name, cannot use 0x20 (space character) in a plugin name"

Server log

org.bukkit.plugin.InvalidDescriptionException: Restricted name, cannot use 0x20 (space character) in a plugin name.

What it means

Paper does not allow spaces in plugin names and refuses to load the plugin. Older Spigot versions quietly replaced spaces with underscores, so a plugin that worked years ago can fail now.

What causes it

The name in plugin.yml contains a space.

How to fix it

Before

name: My Plugin

After

name: MyPlugin

Other plugins find yours by this name in their depend lists, so tell them if you change it.

Check the whole file. Paste your plugin.yml or drop the plugin jar into Permistry. It finds this and about 40 other problems, and explains each one in plain words. It runs in your browser and nothing is uploaded.

Open the checker