I think if you need to load a custom directory URL outside of jme context use DesktopAssetManager(URL configFileURL) constructor(it registers locators for you based on the assets extensions inside that dir) like what jme LegacyApplication does :
But i have no idea what does the AssetConfigURL key refers to in the AppSettings HashMap Manager…
The j3o loader is always registered by default unless you get your asset manager from a very strange place.
Edit: ah, I see you made your own… but why?
Edit 2: if you pass ‘true’ to new DesktopAssetManager() then it will register the default loaders (j3o won’t be the only one you are missing)… but I still wonder why you needed to instantiate your own.