Getting “Unreachable Page” Error When Using Fetch As Google

I've never encountered this error before and haven't done anything differently on this site than any other.
No matter which URL I submit into Fetch as Google, I get the "unreachable page" error.
My robots.txt and sitemap seem fine. Has anyone encountered this before? I can't see it being a firewall blocking the IP because I'm using the same hosting account for this site as many others without error.

See original post by BradySEO