update iCloud settings - Mac Catalina

Please please please can someone help with this ..... its driving us nuts.... we have a 2014 iMac running OS Catalina - this is last release I believe that this Mac will run.

We had to flatten it last week as it threw a bit of a wobbler and reinstall Catalina which went fine..... but we have an issue with iCloud and Apple ID - in System Preferences it keeps asking us to update the Apple ID Settings - we click " continue" and it comes up with Update AppleID settings - some account services will not be available until you sign in again. <<continue>> - we click continue and it goes back to Update Apple ID Settings ! - We are already signed in to iCloud with our Apple ID . If we click on iCloud - we noticed that the Keychain checkbox is unticked. When we tick it, it says that our Apple ID needs to be updated to allow end to end encryption. We enter our password, it thinks about it and then unticks Keychain and we go back to square 1 in an endless loop. We have tried logging out of Apple ID, and then rebooting and logging back in - but its made no difference whatsoever..... has anyone found an answer to this bug ? How do we get it to tick key chain and keep it ticked and stop asking us to Update AppleID? - we have 3 other Macs and a Mini all running the same Software release (also all 2014) and none of these have this issue ?

Occasionally it asks us for login password for the Mac and the key chain stays ticked but is greyed out but after a little while it prompts for the Apple ID password again and the key chain unticks itself ....

We've tried changing our AppleID password - but that's not worked either.

Thanking someone in advance with far more knowledge than us to help us with this !!