REPLYING TO MYSELF--FAILURE FIXED
I asked my Sure Support people at the host and they said
"We checked your WordPress installation. In this particular case, the "BackWPup" WordPress plug-in seems to be causing the issue. You should rename the "backwpup" folder storing this plugin's files (to "backwpup2" for example) in the /wp-content/plugins directory. This will basically disable the "BackWPup" allowing you to log inside the WordPress Administrative back end (your website will start working as well)."
I did this and they are correct. Site works and I seem to have full access to back end.
I wonder if there was some mention of this on the UPGRADE page that I did not see, or if there is NO mention of it, or any of the other problems, on the upgrade page.