Again, thanks so much for your help, Bob.
I installed the new theme and tried regenerating thumbnails by switching the theme and then switching back to another theme in the site settings. The Mercury 2.1.0.0 theme is being used.
Unfortunately, the spinner still persists on the server when viewed using Chome. Another browser (Edge) does not show the spinner.
I also tried to delete all of the files generated by Publii and re-uploaded to the server again. The spinner still persists.
What I am doing wrong here?