I got it.
Or it seemed as though I had. In spite of up to date AVG and Malwarebytes, on Saturday night I suddenly had a security alert saying that I had a problem.
It was a fake alert. It turned out to be relatively easy to get rid of it, but it took more or less a day's effort to find that out.
If you should get it, GeekPolice and GeeksToGo are very helpful.