Joomla 1.5.23, CB 1.7, CB Subs 1.2.2

Our membership plans are set to expire with no grace period.

Users with expired or unsubscribed plans are informed a membership is needed when they attempt to login and they are directed to the registration page to purchase a plan.

When they attempt to purchase a plan, they get an error 'account already exists', 'Registration Complete' and 'You may now login' without getting to the basket or invoice pages in order to purchase a plan (and they still cannot log in to the account despite these messages)

Is there something in the plan configuration we are missing that would enable users to come back, reactivate their accounts and purchase new plans?

Thank you