Ah, looks like it's the cloudflare DNS issue:

https://twitter.com/StackStatus/status/609296474015444992

http://www.cloudflarestatus.com/incidents/br8t7ds3mhh6

https://news.ycombinator.com/item?id=9704843

(Nothing to see here)

Could be a fallout of DNS issues at CloudFlare: https://www.cloudflarestatus.com/