After you imported the certifictae to the controller, you should configure the controller to use the new certificate for webserver on the controller.
In GUI
Management > General > WebUI Management Authentication Method > server certificate (select your new certificate)
CLI:
(MC1-Sunnyvale-3600) (config)# web-server switch-cert <name of the cert>
The CA that issued the new certificate should also be a trusted CA in the certificate store of the client in order to get rid of the error
Regards,
Sathya