I would check the Application Log, which is under the Guest/Onboard part of ClearPass, down to the bottom under the section Support -> Application Log:
In many cases the Application Log will show you what is happening and why requests are failing.
Also check that there is no proxy of firewall in between that may require specific authentication. A 403 error code means that something is actively denying the request from your client to ClearPass. I would start to see if ClearPass Onboard is that something, by checking the Application Log.
Herman