Please help with .mp4's and .m4v's on Windows 10

Hi,

Over the years, I have encoded many videos using Handbrake, to stream over DNLA to my Roku player. On reinstalling Windows, I decided not to install iTunes, which I never use otherwise. I installed the k-lite codec pack and am able to get most of my videos (.mpg, .dvi) to play through Windows Media Player, but none of the .mp4 or .m4v videos will play. They show a "gray" box in the filmstrip thumbnail, and although WMP opens, it just sits there, playing neither audio nor video. I have spent HOURS reading the forum and trying to figure this out. Now I am throwing myself on the mercy of anybody who can make more sense of this than I. (Can't resist saying how much I HATE how difficult this is. I am trying to edit a slideshow, not engineer a software environment....)

As requested on the FAQ, I have generated logs both for how many codecs I have installed, and the information files on the videos that won't play, but I can't figure out how to post files to the forum. If somebody could give me a tip, I will post more information immediately.

Thanks in advance.

WMP uses Media Foundation for playback of .mp4 files. This allows using the better codecs from the codec pack:
Codec Tweak Tool > Media Foundation > disable for .mp4

To fix the thumbnails:
Codec Tweak Tool > Thumbnails > Options > Clear thumbnail cache