Login black screen of death

A couple of days ago, I started up my old Macbook pro (MacBook Pro (Retina, 13-inch, Late 2013, High Sierra 10.13.6) -- everything looked normal. I clicked on my user icon and entered password. The progress bar advanced about 90% and then stopped. For a long, long time. I finally held down the power button until it shut down, and then restarted. This time, when I tried to log in, the progress bar advanced about 50% and then the machine went to the black screen of death -- black screen with a white cursor. I can move the cursor around, but nothing happens.

I tried all the possible solutions I can find on the web with no joy. As last resort, I went into disk utility, created a new volume, and installed MacOS on the new volume. This copy of the OS works perfectly well.

So now I can reproduce the problem by booting on the old volume, or I can boot normally onto the new volume.

Does anybody have any ideas about this? I'd like to know what happened, even though I do have a fix (if you call starting over from scratch a fix.)

Thanks.