I've been using for Lightroom for years and I constantly battle the above issue, but lately it is a lot worse. I have cleared cache, reinstalled, and I'm on the latest CC version. I have many gopro and apple files, but no video file will import correctly. The files open correctly in VLC and I don't see anything unique about the encoding such as file type, fps, new camera. All similar files have imported correctly in the past.
I do not have quicktime installed due to the security announcement.
What format do the files have?
Read here about supported formats: Supported file formats in Lightroom
You did remove Quicktime. Reade here about the impact: