Redownload the codec zip. A partial download will cause MX Player to reject it without an error message.
The ultimate open-source player for Windows. It plays Xvid, DivX, MKV, and almost any video format without needing external codec packs. xvid video codec for mx player 2021 windows 10 link
Go to the official Xvid site and download the .exe file for Windows. Run the file: Double-click the downloaded installer. Redownload the codec zip