SDV files are notorious for header corruption. Use a hex editor (like HxD) to check the first four bytes. An SDV file should start with hex 53 44 56 00 (S D V NULL). If it doesn't, you need repair software, not a player.
If you haven't tried an SDV-compatible video player yet, you are missing out. The playback speed and scaling are actually insane. 🚀 sdv video player