Recurrent error during sync: "The certificate for this server is invalid"
Most likely is that you need to tell your iPhone to clear this "safe-browsing" data, and then recreate it. 

https://www.intego.com/mac-security-blog/google-safe-browsing-data-syncs-to-ios-devices-via-itunes/

On your iPhone device, go to Settings > Safari, then look for the Fraud Warning slider and turn it off. Then resync the iPhone. Turn the Fraud Warning back on and synce your iPhone again and hopefully that will clear up the warning.

Less likely: you might be able to clear this up by resetting the iTunes Store cache under Edit > Preferences > Advanced. 

https://discussions.apple.com/thread/7843167?start=0&amp;tstart=0