Yeah, now I recall that on some systems with AppArmor/SELinux or on OpenBSD tor as other daemons has restictions applied on the paths accessible for them. Try to put your unix socket into tor's DataDirectory. At least it works for me on OpenBSD.
David Goulet:
I use extensively Unix socket for HS so I can tell you it works but the permissions are can be tricky.
I'm also using Unix sockets a lot and can say that 'rw-' is enough for plain Linux. In other case note above.
-- Ivan Markin