A bug in how the server handled start and end arguments for MP4 streaming, which prevented videos from playing correctly on iOS devices.
If you are a developer hosting this file, ensure your server supports "Byte-Range Requests," which allows mobile devices to request specific parts of the MP4 file for playback. 521 mp4
If "521" is simply a partial filename you are trying to open, follow these standard steps: A bug in how the server handled start