: VirtualDub requires specific Video for Windows (VFW) compatible codecs to function. Standard DirectShow codecs used by modern players like Windows Media Player are often incompatible.
The newer VirtualDub2 includes built-in support for more modern containers and codecs, which often bypasses the initialization issues found in older variants. : Unable To Initialize The Output Video Codec Virtualdub
Navigate to Video -> Compression and ensure the desired codec is listed and selected. : VirtualDub requires specific Video for Windows (VFW)