There is a XQuartz issue following minor updates of MacOS Sierra both on Mac Pros and on MacBook Pros. I.e, it is possible to run and access X applications on a host and even to connect to X applications running on other hosts on the LAN via xterm, but not also via Xnest. Trying to do so, Xnest attempts always result in a "Connection refused" issue, whether Xnest is started directly on a LAN host or via a proxy server on the LAN from a remote host, for example via the Internet, unless XQuartz is re-installed following the MacOS update and the user logs out and in again.
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This bug has been migrated to freedesktop.org's GitLab instance and has been closed from further activity.
You can subscribe and participate further through the new bug through this link to our GitLab instance: https://gitlab.freedesktop.org/xorg/xserver/issues/752.