Bug: Sometimes plugin with spigot 1.12.2 throws exception IllegalArgumentException on server starts
Fix: Workaround with concurrent calls to definePackage in spigot PluginClassLoader (exception IllegalArgumentException during a call to PluginManager.registerEvents)
Created issue: https://hub.spigotmc.org/jira/browse/SPIGOT-3734

Edit:
@md_5 has fixed this bug. If you updated your spigot 1.12.2 to last version then you can skip this LigthAPI version and use 3.2.4 X.12.0