User Login Issue

After upgrading, customers are not able to login to their accounts. It acts as if they don't have an account.

Even with their account in the database, it allows them to sign up for a new account using the email address in the database, so on the backend it shows them twice.

However, if I got to their original account via admin, and deactivate, save, and activate and save their account it begins working.

Any idea how to fix all users in the database? (It's thousands)