Safari 14.0.3 opening bug

Since the update, I get asked to "Confirm the file you want to load for bookmark "xxx.html" over and over again. For multiple pages I haven't even visited in well over a year! When I cancel each one, Safari goes on to open a huge number of tabs, more than I'd have open in a week of sessions! It opens a small popup window with a url I haven't been to for over a decade (now defunct, fwiw).

Most of the tabs appear to be more recent history. Huge resources drain, slows the MBP to a crawl, even freezes it.

I have to force quit to get my machine back.

What to do?