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.
![](https://d1uyvls174j03l.cloudfront.net/extremenetworks-us/attachment/RackMultipart20150610-19942-hfleyf-Screen_Shot_2015-06-10_at_9.45.39_AM_inline.png)
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.)
![1117d1ee214744a09d7bdaa3910a68f6_RackMultipart20150610-10081-8egyoe-CertDiags_inline.png 1117d1ee214744a09d7bdaa3910a68f6_RackMultipart20150610-10081-8egyoe-CertDiags_inline.png](/t5/image/serverpage/image-id/5804i6F933912A6F65E1D/image-size/large?v=v2&px=999)
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.