This morning (the next day after I posted the below) it seems to be working normally again. I had tried rebooting several times before with no effect, so have no idea what changed, but when I went to Show Applications in the favorites bar this morning and clicked on the Ubuntu Software icon, it worked the way it always did. It took a long time to load the software list but no longer than in the past. I had tried the suggestion in the answers to related questions with no apparent effect.
Snap store does not come up when I click on its shortcut in favorites, nor when I click on it in the Show Applications window. Right clicking, then clicking on "new window" also does nothing. The little "I'm busy" twirly thing shows up for about 20s, then nothing.
However, when I right-click in Show Applications and select "show details", Snap Store DOES come up with a little text box saying "no details available" or something to that effect. Once I close that text box, Snap Store seems to be working normally.
I've tried all the suggestions that came up here to the same question (but without the except... part). None of those procedures made a difference.
This is with Ubuntu 20.04 on a 6 year or so old HP Pavilion quad core AMD 64 laptop. Lots of free disk space. 8 GB of RAM
