Further information - the backup has now started working properly - and I have no idea why.
Upon considerable amounts of testing, I determined that something was indeed stopping the Chrome Bookmarks file from being backed up - on 1 machine only. Just the Bookmarks file was missing - ALL the other files/folders of the directory where this file resides was being backed up.
I changed only one setting on the server under File Baclups: “Default directories to backup:” and set them to “C,D”.
When I re-ran the image backup on the offending machine, the Bookmars file is now included in the backup.