When using rest applications we see repeated forms based failures before authenticating with Kerberos.
Release : 15.8
This is due to the order authentication mechanisms exist in the SpringSecurityContext.xml file
The forms based authentication failures can be ignored.
To prevent them all together you would need to modify the springsecuritycontext.xml to remove or move the forms based authentication modules. However this will prevent the DLP Administrator account from being used.
For more details for the spring security context please see(How to Configure AD User Login Authentication)