Suddenly cannot transfer movies from PC to phone

This could be a phone issue but it could also be a PC issue, I do not know but I suspect PC because Windows Explorer hangs every now and then when I've got the phone plugged into the PC. Phone is a LG G6, drivers have been updated.

Copying movie files from my PC to the phone to used to work just fine until today. From my PC I can see all the files in my phone using the USB, so it's being recognized. I've tried multiple USB cables, turned off my AV, tried different movie formats, restarted the PC, I can delete phone files from my PC, I can transfer pictures and other files from my PC to the phone. But it will not transfer MP4, MKV, AVI from my PC. It just hangs for a few minutes and says it cannot copy the files.

Absolutely baffled.

Is it possible that your phone doesn't have enough room for any more movies? Movies take up a lot more space than pictures and most other files, which could explain why those items can copy but movies can't. Check the amount of free space on your phone, and then check the size of the movie. As a test, see if a very short movie (one or two seconds long) will copy to the phone.

Are you copying to phone or SD card?

I agree it could be full memory either on phone or memory card or maybe the movies are very big in sizes so his 8GB ram is not enough while performing other tasks.
BTW, what is that Black Label 7 x64?