That IP is a virtual interface and used to be default for the Cisco WLC webauthentication, but not necessarily so.. Is it a brand new Cisco WLC then it might be 192.0.2.1. Try changing to that IP in your weblogin page on Clearpass.
If thats not it, then - do you have any more information to us?
Anothing thing to consider is to go the route of doing MAB webauth. That way the Clearpass server will use Radius CoA to change the acl of the client to authenticated instead of the client accessing the WLC directly.