If "521" is simply a partial filename you are trying to open, follow these standard steps:

A patch was released to ensure the server correctly processed request headers for seeking within a video file. If you are seeing errors related to this, ensure your web server software is updated to a modern version. 2. WhatsApp and Distributed Systems Logging

If you have sound but no video (or vice versa), try using a versatile player like VLC Media Player which supports almost all MP4 encoding formats.

If the video stops abruptly, it may be a partial download. Try re-downloading the file.

Could you clarify if you are seeing this as a , a specific bug report , or a file name you need help opening?