With the 5.0.0 release a lot of the plugin's internals have be rewritten. If you are upgrading from the 4.1.0 please look at the spigot page for the plugin as there are instructions there.
Here are some of the new features:

• Hot bar messages are now supported without an external plugin
• Completely configurable item names, includes support for nbt so that mob spawners and custom items can be supported
• Completely overhauled language files
  • Support for future translation support
• Supports modern versions
• New arenas system
• There are many more features and the spigot page will be updated as more are added!