"This certificate was signed by an untrusted issuer" problem after updating to El Capitan Most of the https connections returns that. Most of them is signed by Symantec. But not all. I tried to google the problem, but with no luck, seems like I only the one who experiencing this problem.

I checked apple's certificate with https://www.sslshopper.com/ssl-checker.html and all seem to be correct.

Some of the blocked sites can't be reached by Chrome due to HSTS: 


This happens when I open iTunes: 
After clicking "continue" there are appears more such windows, most of them on apple.com domain just with different subdomains.

Please, help.
Same issue. It is certificate related on your own mac. 
Your machine has expired certificates which can be deleted using: - applications > utilities > keychain Access.
- Select a) Keychain: Login and b) Category: Certificates
- Scroll each certificate and if it has expired delete it.

I found the answer including instruction and images via link
http://tiffanybbrown.com/2015/10/cant-connect-to-itunes-app-store-https-sites-el-capitan-upgrade/