I can browse to https://safebrowsing.clients.google.com/ but it just redirects me to Google's home page.
iTunes 220.127.116.11, Windows 8.1
I saw this same error on multiple occasions in iTunes 18.104.22.168 on Windows 10 when I tried to sync my iPhone.
I just upgraded to iTunes 12.6, and I no longer see that error.
This is an issue with iTunes 12.5.5 that needs fixing by Apple.
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