Problem displaying previews in Web Contact Sheets

My Web Contact Sheets (using Classic format) display perfectly on my Windows 10 computer, but when I transfer WCS files in their folder to a second WIn 10 computer, the web page displays the thumbnails but does not open the popup Preview image -- instead it displays the Preview at the bottom of the page, beneath a horizontal line.

This problem recurs with all four browsers (FF,Chrome, Explorer, Edge), so there must be a Windows 10 setting that causes the problem.

I have disabled adblockers, updated Win 10 and the browsers. Still they refuse to display the Previews properly.

Any advice greatly appreciated

--s