Lightroom Classic CC; Major Problems with Video Files

I am experiencing multiple problems with MOV files in LR Classic CC 8.1. No video file will play, If I click on one and press play, it jumps me back to the grid view. Sometimes it says "There was an error working with the video File" When this first started happening, I was able to fix it by resetting my file permissions in OSX High Sierra. But, after a couple of days, it started again. I repeated this permission resetting a few times, but now, it no longer solves the problem. I have also tried purging my video cache, resetting LR preferences, and  verifying the catalog file. Nothing helps.

If I click on a folder of video images it takes several minutes just to render the thumbnail previews of each one, if they render at all. Mostly they are just blank thumbnails. An almost bigger problem is when I go to import images from an SD card. If there are any video files included, it takes FOREVER to simply create a preview of the files to be imported - 10 to 20 minutes sometimes.

In prior versions of LR, I had no such issues with video files.

This is super frustrating. I should say that there are no issues with these files outside of LR. They open immediately in Finder or Quicktime.