look in your clearpass guest section, Home » Configuration » Web Logins and then the web login used in this case. there look for Pre-Auth Check in the Login Form section. it is probably set to App Auth - check using Aruba Application Authentication.
i believe it does little harm to disable it if you are also doing the further auth on ClearPass. from an earlier thread i recall it is mainly useful to show extended errors, something which is dificult after the final auth.
if you dont want to disable it you should use the template i mentioned before to make sure the auth request are classified correctly and picked up by the CPPM.