Thank you JohnnyF50. I was bitterly disappointed in Apple’s lack of meaningful support, and the evident creation of problems where none formerly existed. Your suggestion helped me immediately on both the macbook pro and the iMac, neither of which would connect to the Apple store, nor permit certain downloads etc. after I updated all of my phones and tablets to iOS 9.1 (which was another drama that need not be repeated here) and I changed my Apple ID in consequence of the hand-held update setup steps when the MacBook and iMac would not connect to the store.
Your solution was immediately effective and I thank you for the advice. Frustratingly for me, the pc’s could still access the store and everything appeared to work OK, until/unless I was asked for my Apple ID password, and in that case, it refused to connect. Even more frustrating was the fact that I could log into the store with the same password on all of my other Apple devices - just not the macbook nor the iMac.
Kaspersky users, and perhaps others, might also notice that the firewall raises an alert when their apple notebooks or desktops attempt to connect with the Apple Store. The substance of the alert is that Apple appears to be using a different URL that users normally would not see when connecting to the store, and Kaspersky (in my case) warns that the certificate for this URL s not valid (expired) and may not be trusted. On the face of the evidence, it seems that Apple is becoming sloppy in its care for its former dedication to the trouble-free user interfaces upon which it built its otherwise excellent reputation. I offer as evidence of this, that El Capitan has dropped a star in its rating on the iStore “Best New Apps” in only a couple of weeks.