I get this error a lot when I try to follow the “Visit Topic” button in an email notification:
Hmm. Seems to work ok here (on Firefox, mind you), but presumably there’s some voodoo going on with that subdomain that’s (I guess?) part and parcel of the Discourse install. If it keeps happening, we can try and get more forensic…
If I click through via the link that embeds it works fine, but the same version coming from my email doesn’t. Let me try to paste it without the embed widget.
Yeah, try that.
Still works 
I wonder if the developer tools in Safari would shed more light on what’s not happening between you and the server
Hmm, clicking it now works too…
Oh, it looks like it may be browser-specific thing. Link works fine (from the email) on my laptop, but not on my desktop.