I am having the same problem trying to import MP4 video files from my Canon G7X. I know the files are ok because I can play them and edit them with Premier pro, but LR 6.5 refuses to import them.
And I have just tested a couple of .MOV files from my Nikon D800, they would not import either but I know they did work ok with LR6.3 ....seems to be a problem with "dynamic link server" in the latest updates perhaps ?
Sad and glad to hear that I’m not alone on the island. I even set up another folder on my desktop and copied some video files to that folder. Then used the steps from within Lightroom to Import and got the same errors.
I also checked all of the Adobe Applications file contents - for some reason the Dynamic Link Server info does not look right - version #? - and is not located in the same place as the other application’s contents on my Mac OS.
Also, oddly, some video files did make it to LR while others (too many) have not. Because I needed to post some work, I used another application - not Adobe - to process everything. So I have not lost any assets on my OS either.
I might give support a call, when I have some spare time. If I come up with anything, I’ll be sure to let you know, too.