For anyone else who gets this after upgrading to Magento 2.4.3 or above.
We had the same issue. Fix for us was:
Temporarily disable forced passwords
php bin/magento config:set admin/security/password_is_forced 0
php bin/magento cache:flush
In the MAP we had to go to:
Stores > Config > Advanced | System > Security
Set Max Session Size in Admin to
This gave a warning, but saved fine.
Then flushed cache.
We could then re-enable force password change
php bin/magento config:set admin/security/password_is_forced 1
Stores > Config > Advanced | Admin > Security
Set Password Change to
Then flush cache.