Export as MP4, but no media player plays the file

I've used Premiere Pro for years, and I've never changed the export settings. Always H264 at YouTube Full HD.

But today, for the first time ever, aftedr exporting, no media player will play the file. It tells me "Choose something else to play. This item is in a format we don't support. 0xc00d36b4"

Now, I've searched the internet about this code, it says you have to download a codec, but the file is mp4, and every player I have, I've used many times over and never had this issue until today. I checked and made sure I am up to date

Windows is also up to date 

And of course, system specs

Any help to get this back to working properly would be so appreciated. Customer service chat support left me without even a greeting TWO TIMES, even after I see that the agent has been connected - 10 plus minutes later, nothing. Hopefully the community will be able to help ♥