I was trying to install xQuartz on macOS Sierra beta 10.12 developer preview and it would get stuck installing near the end. Also, for people that already had it installed, when they tried to ssh -x something in terminal and run something that need X11 forwarding, this appears
/private/tmp/com.apple.launchd.ctkLYov2Id/org.macosforge.xquartz: unknown host. (nodename nor servname provided, or not known)
I hope this gets fixed ASAP!
Thank you for reading,
The "stuck installing near the end" is likely font caching taking a while (as it does). You just need to be patient and wait for it to finish.
The ssh bug is an OS bug and already has a radar. Please remember your NDA and report bugs to http://bugreport.apple.com.