Have finally solved this, courtesy of Lightroom Classic: Videos don't play and don't import | Photoshop Family Customer Community
Deleting the contents of these three folders worked:
Sadly, this does not fix the problem on my Mac (running High Sierra 10.13.4) I've deleted all of the dynamic files and am still unable to import any video (MP4, MOV, AVi....). All of the files play in Quicktime and the new Lightroom CC....but not in Lightroom Classic
This is ridiculous and from the forum has been a problem over many versions of Lightroom. Luckily I've only been affected in the last couple of months. I have found that sometimes if I shut down my Windows 10 PC, and restart it and Lightroom, it is suddenly able to import videos.
Today, it refused to import jpg files, telling me that there is either it is not writable(!) or there is not enough space (only 129GB!).
What is the point of software that is as irritating as this. Do they want us to keep subscribing?
I just tried again (having changed nothing - just wasted some time posting this grumble) and it has worked. If only video imports were as easy. It still needs sorting out permanently by Adobe.