Could it be that you locked down application level access the the ClearPass Admin UI?
That is configured: Administration » Server Manager » Server Configuration > <your clearpass node> .> Network > Application Access control.
If your deployment is locked down (which is good) and administrators come in from an unauthorized IP address (maybe the network was renumbered), it could be that you see this 404 error.
If you still have access to the console (appadmin), you can reset the application access control from there:
[appadmin@cppm1-pub]# system apps-access-reset
Policy Manager application access is restored
Stopping httpd: [ OK ]
Starting httpd: [ OK ]
After that, you should have access again.