If the proprietary header is strictly non-standard, FFmpeg may fail to locate the stream start point. In such cases, the probesize and analyzeduration options may need to be increased to force FFmpeg to look deeper into the file structure:
This is the most reliable way to get a high-quality MP4 file. Since the .anydesk file can only be played in the AnyDesk software, you can "record the player" using a screen capture tool. convert anydesk video to mp4