Hi Mark,
There’s a known issue, observed by me and at least one other user, where the client occasionally fails to connect. In my experience, this always seems to happen immediately after a macOS restart, but I’m not certain that’s a required precondition. I’ve found that the only solution is to restart the Mac again, after which the client connects normally.
This has been a difficult issue to investigate, because it doesn’t happen very often. I think you may be right that it’s related to IPv6 somehow, but I don’t know if the issue lies in the OS, ZeroMQ, or somewhere else. I don’t think I’ve tried switching from “localhost” to “127.0.0.1” as a workaround, but I will the next time it happens to me.
For now, can you see if restarting your Mac resolves the problem? If it does, then I suspect you’re experiencing the same issue.
Thanks,
Chris