Why is the symlink I created inside an application container to allow for external storage not working?
For some reason, symlinking a part of the folder inside of the Containers folder doesn't work. If you want to symlink, you have to symlink the whole folder, i.e. com.vemedio.osx.Instacast

See https://snipt.net/certainlyakey/os-x-sandboxed-apps-and-symlinks-problem-solution/ for reference.