Why do I get an 404 Invalid redirect_uri when using my own App?

Did you exactly follow the instructions in the documentation? This error suggest you didn't added the given url on the Plugin Settings page exactly to the OAuth redirect URIs in the App console.

If you would like to use your own App, only add your url to the OAuth redirect URIs in the App console if you have a valid SSL certificate installed and you are visiting your WordPress Dashboard via https://. Otherwise you have to add our url ('https://www.florisdeleeuw.nl:443/out-of-the-box/index.php') to the OAuth redirect URIs