Verification failed - unknown error

I've read most everything I find on the various flavors of unexplained "Verification failed" problems but don't find anything that quite matches my scenario. I am looking for informed advice.

My problem involves a long term iCloud account with known, valid credentials, one logged in iphone XR and one logged in iPad (an old ipad4 - ver 10.3.3). There are no recent known account/upgrade/update changes. Both Apple devices have "Find My" enabled via conventional configuration. The iCloud account credentials have always worked as expected everywhere, including log in via several Windows desktops using browsers and/or iCloud/iTunes desktop apps until yesterday when I began getting "Sign in to iCloud" password prompts on the iPad in response to various selection options. The iPad is logged in and has normal access to the Photos, Calendar, Messages apps, etc. I can log in with the account credentials via any of the desktop login routes with the iPad providing the required 2FA codes, but response to the iPad password prompts is always one of the verification failed errors. This includes the "Sign Out" iCloud selection, which requires the password to turn off "Find My". This verification failed event means I cannot sign the iPad out.

I have done soft/hard resets, verified the WiFi signal, etc. but don't want to tinker with Resets or password changing until I can understand what is happening.

Any help will be appreciated.