If the file size is 0KB, the download failed or was deleted from the server.
Use VLC Media Player (it plays almost any .mp4).
Open your File Manager → Internal Storage → WhatsApp → Media → WhatsApp Video. VID-20181029-WA0107.mp4
Look back at your WhatsApp chat history from October 29, 2018 .
If you don't want to open the file but need to know what it is: If the file size is 0KB, the download
Open the Photos app → Albums → Shared from Cast / WhatsApp.
The filename is a standard system name generated by WhatsApp. It indicates the video was received or saved on October 29, 2018 . If the file size is 0KB
📎 VID (Video) - 20181029 (Year/Month/Day) - WA (WhatsApp) - 0107 (Unique ID).