I've got the same issue: after turning on devise confirmations previously created accounts are unable to login.
The reason is here:
def confirmation_period_valid?
self.class.allow_unconfirmed_access_for.nil? || (confirmation_sent_at && confirmation_sent_at.utc >= self.class.allow_unconfirmed_access_for.ago)
end
Old accounts have confirmation_sent_at set to nil, that's why they are unable to log in.
One solution is to force confirmation_sent_at like that:
update users set confirmation_sent_at=created_at where confirmation_sent_at is NULL;
You can do it manually, or create a migration.