Token Confusion

It sounds like your own app is probably registered for the "app folder" access type, while the API v2 Explorer has the "full Dropbox" access type. Apps with the "app folder" access type can only access the contents of the special app folder created for them (by default at "/Apps/<app name/" for accounts using the English locale). You can find more information on this here. You can check the access type for your app on the App Console.

Since API calls using access tokens for apps with the app folder access type are rooted inside the app folder, attempting to access a file at a path outside the app folder will fail.

You should either just reference items inside the app folder, using a path relative to the app folder, or, if you need to access any location in your account, you should instead register an app with the "full Dropbox" access type.