those codec packs can be a devil.
Since VLC works I would keep using that. You can try uninstalling the K-Lite codec pack and look for others, but it's a minefield out there with codecs.
Hi, I have been playing mp4 files with window media player by installing the k-lit codec pack. This has worked beautifully for me.
However, a couple of weeks back, I tried playing one of my files and there was no video just audio. This has extended to media player classic. I'm thinking it might be a codec problem because vlc can still play the files with no problem whatsoever. I can't recall changing any of the codec settings.