When I started my app as a Web Start app, I was greeted with these two fellows:
The error message seems obvious. Java is unable to find a resource. To be more precisely, it fails to find an fxml document. The usual diagnosis is... The resource is not there. Or... you are doing that getResources() wrong.
To put a long story short. Not being able to find the resource here means not being able to access it. My Web Start app was sandboxed. As soon as I requested all_permissions, everything was fine.
So, for example, in a NetBeans project just request unrestricted access. Wham!