I'd start by posting the output of zonecfg -z <zone> info ipadm show-addr and an example of the command you're trying to run. That might provide some clues.
I have the same problem.
I believe this message may arise as a result of not executing as root (or other privileged user). When I do this I can see this in
Aug 19 12:50:17 testhost1 /sbin/dhcpagent: [ID 928213 daemon.warning] ipc_event: privileged ipc command (extend) attempted on net2 Aug 19 12:55:59 testhost1 /sbin/dhcpagent: [ID 928213 daemon.warning] ipc_event: privileged ipc command (drop) attempted on net2 Aug 19 12:57:58 testhost1 /sbin/dhcpagent: [ID 928213 daemon.warning] ipc_event: privileged ipc command (release) attempted on net2
So instead of getting a sensible error message back from
ipadmcommand you get "Communication with dhcpagent failed instead". That sucks!
However I also get the error sometimes even if I'm root. It seems to me that the message "Communication with dhcpagent failed" can mean all sorts of things.
Like you I'm also searching for a solution that will get me out of the problem. Like you I have also previously been through the whole power cycling circus when I have this error (and that works!) but as you say it is really not a solution.
Now I'm eager to get to the bottom of this.
I've managed to get away with "only" booting the zone that had the problem. Didn't have to power cycle.
Not a solution though.