login.keychain-db gets renamed after OS update. iCloud sync then resets it each time.

You wrote -

I did some research here and found a real useful post which said that the system likely made a copy called login_renamed_1.keychain-db and created a new login.keychain-db. Sure enough, when I look in ~/Library/Keychains I do see that on the day I did the OS update, there was a new login file created and my existing one was moved to this _renamed file. Uhhh, why???

If you have forgotten the password completely and you know iCloud verification code , all details ( mail addresses) of Apple ID , iCloud ID , webmail ID and respective passwords , base station or wireless network password for air port utility .

Shut down the system through apple logo , press command , R and power key simultaneously , release the keys when apple logo appears let the bar be completed , select the language as English .

Take cursor on top menu bar > click on utilities > click on terminal in the terminal window execute a command resetpassword and click on enter key .

Select your user account , you have to press forcefully on the trackpad to get it selected .

Some fields will appear .

New password ……

Verify password ……

Hint ……

Fill up the credentials ( enter a new password ) and click on next then you have to click on restart .

The Mac will restart but you will see pop ups as this Mac can’t connect to iCloud because a problem with your mail address open iCloud preferences for problem.

You will see two options ( a ) Later ( b) iCloud preferences

And on the top right corner web mail ( could be gmail ) pop up will be there stating google password required enter your password for gmail address and showing close and continue .

Open system preferences > click on iCloud you will see the box for keychain is unchecked, so check mark the box a small pop - up will come on the screen after a while to enter iCloud verification code so enter it .

Make sure your registered mobile is opened , after a while six digit code will come through sms on the mobile .Enter the six digit code on the next pop up .

You will see iCloud Keychain is setting up and a small circle is rotating the box for keychain is configured with iCloud server .

Now , on the top right corner of the desktop a google pop up is showing , click on continue a window will appear enter your gmail password so that is also configured .

Open airport utility through spotlight , click on base station you will see that password is empty , click on edit a pop up window will appear enter the base station or wireless network password .

Keep the box checked for remember this password in my keychain .

Open safari and you have to reset the homepage once again ( open safari preferences click on General , enter the search engine link and take cursor on history > click on home close safari a pop up window will appear change home page , click on it ) .

Open mail > window > connection doctor shows green dot in front of IMAP / SMTP both for gmail and iCloud account .

Lastly shut down the system when login window appears you can enter admin name and " new password " in the fields .

Note : If you enter in user library > keychains you will see login keychain is renamed and its a confirmation that login password has been changed and always be careful once login password is changed don’t delete login_rename …Keychain-db from user library > keychain folder , if it is deleted the keychain application will be corrupted .

If you open keychain access and click on file > click on add keychain then an empty keychain is created and you can add some important data like passwords for any web sites in it .

But this has no connection with login.keychain.db and they can not be interchanged .In case if it is done keychain access application can get corrupt , sometimes the user has to install OS , so it is to be avoided .And if you send a feed back to apple that is well .

By default, your keychain password is the same as your user password (the password you use to log in to the computer).

And you can login to the Mac but when you open system preferences > iCloud sign in with your Apple ID and there is a problem in iCloud Keychain .

See this article Get help with iCloud Keychain - Apple Support

See the last paragraph of this article If you enter your iCloud Security Code incorrectly too many times - Apple Support