and others may be interested in learning how your problem was fixed.
It appears that, rather than get my problem fixed, the Apple technician just made it the same as Tacit's. Since I last posted I continue getting, every couple of days, a dialogue box saying that my Mac can't access iCloud. If I sign in the problem goes away for a couple more days.
So, all that happened with Apple support, is that the iCloud now recognizes my Apple ID.
iMac (Retina 5K, 27", 2017), 3.4 GHz Intel Core i5, 8GB RAM, 2400 MHz DDR4
OS High Sierra 10.13.6
Canon MX710 Printer
Epson Perfection V500 Photo Scanner
Time Machine on 320GB OWC Mercury OTG Pro
Super Duper on 500GB OWC Mercury OTG Pro