"This certificate was signed by an untrusted issuer" problem after updating to El Capitan
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/