Thank you, r@vast, but pinned folders are not the same as recently visited folders. Your advice is not relevant to my issue.
Today, I went to Rescue Center in Avast Cleanup and clicked 'Undo' next to 'Broken shortcut'. To my surprise, the quick access folders re-appeared. But, unfortunately, disappeared when I re-started the computer. Subsequently, I uninstalled Avast Cleanup, added folders to Quick Access, restarted the computer and the folder are still there.
So, no Avast Cleanup for me. Pity, because I like the program.