This happens when your password was auto-generated for you rather than chosen by you — for example, a family/account-member login the primary account holder created without setting a password, or an account staff created on your behalf. Until you set your own password on /change_password, every other page redirects you back there. Enter your (temporary) old password, choose a new one twice (minimum 8 characters), and submit; you're then free to use the rest of the site.