apolaine’s trick worked for me.
just to rename the plugins folder in wp-content (I rename mine to plugins_). It kills the plugins, which enables you to log in and upgrade. Then rename it back again to plugins.
All of my plugins were deactivated. I don’t get it.
I’m Surprised this problem is not addressed by WP. In 3 1/2 years I’ve never encountered anything like this. There should be a note about the problem in the install instructions.
It took 24 hours to research and solve.