Whenever you restart your phone, not unlock it for several days or introducing footprint is made incorrectly (eg by having wet fingers), asks for the security key. Moreover, this key is required to set more things id like touch add or remove tracks. The only solution is to format and if you "find my iphone" enabled and will not let you, you deactivate from iCloud.com. If you could otherwise access to the terminal, without the need of the footprint or the key, it would not be a barrier to taking security seriously. Greetings.