I understand that you will not be able to hit that webapp. Could you please let me know if ARACHNI is capable of logging in to the application using spring security. Also, let me know if the format of the JSON being used to authenticate is valid.
Unfortunately, like I said, I can't know without access. It seems like Arachni can't find the login form and I need to debug this myself to see why.
I'd suggest that you try using the login_script plugin instead as that'll give you more control over the login process.