Hello Vakhtang,
This is likely a cert error coming from the NAC appliance itself. Since the client has not loaded the NAC's certificate, or you have not loaded the NAC with a trusted certificate, say from an external cert provider such as Verisign for example. See this happens in one of my lab setups.
My NAC's IP is 10.0.0.98 as see below.
You can verify what certficates are in play by right clicking on the NAC IP in NAC manager, select Webview -> Select Certificate Diagnostics. See mine below. It's from our company, so Google does not know it's a valid certificate, as we are not also a certificate authority, and your browser has not installed it (this may be impractical for Portal environments.)
I would proceed as proof of concept.
If this fails, then I would open up a Ticket with the GTAC so we can pursue offline.