M4V Import dead? Can't even view them now - Help!

Did something change with Windows over the past year or so where .m4v files no longer play?

For many years I've been happily exporting my Premiere Pro-edited master files as .m4v files (not muxed, ie separate .m4v for video, .wav file for the audio). I've done hundreds of those. I always would spot check them to verify that they play fine on Windows, and would import well on PPro again should I ever need to.

About 2 years ago, I've switched to exporting my masters as regular muxed MP4 (.mp4) files and pretty much just forgot about the old .m4v files. Now it has come time for me to go back and re-encode all those hundreds of .m4v masters to various formats.. AND THE THINGS NO LONGER WORK.

I cannot import any of those .m4v files AT ALL into PPro 2018 (already uninstalled 2017). Again, those files were MADE by me in PPro (all the way back in CS 4, CS 5.5, CS6, and I believe the last one I exported as m4v was in CC 2017). They always worked and played fine over the years I would test them.

No more. Now when I try to import any of these files, PPro just freezes and stops responding. Same with Media Encoder, those files simply no longer work at all. And I can't even view them in any Windows player, so I have to suspect it's a Windows change that I missed.

I'm panicking a little here. All these masters - work I've done for my clients over the years - no longer can be viewed on Windows Media Player, or even VLC Player. Installed old Quicktime 7 hoping it would fix it since .m4v was originally an Apple thing, but that didn't help.

The ONLY way to view those m4v video files now are with the Media Player Classic, and even then things tend to freeze up after a while.

I NEED to be able to import those files into Premiere and work with them.

I've renamed the extension to every conceivable one such as .mp4, , .m4ts, .mts, and others. Nothings works. I've tried many different files, just in case some were just corrupted somehow. Didn't work.

This is on a clean Windows 10 Pro custom machine, I re-installed the OS just in case the problem was a corrupted Windows.

i-7 3939K, 32 GB RAM, 4 drives (OS, sources, caches, exports), dual GPU setup with NVIDIA 780i,

This is a big problem for my work and I really hope someone might have an idea of what happened here.

Thank you!