The system is now working. I resolved the issue by resetting the GFI.
As I mentioned, this morning, about 1/2 the zones were working. But, by this afternoon, none of the zones were working. I checked the contacts and discovered there was no power to the terminals. After resetting the GFI, all zones were functional. But, here's what was strange: even when nothing was working, the Iro power light was always on, and the blue status light would come on each time I tried to manually start a zone. So it appeared the Iro had power even though the zones were not operating.
Does the Iro have an internal battery with some kind of trickle charge? If the circuit had been tripped off, could there have been enough internal charge to run some zones but not all of them? And, if so, shouldn't the Iro communicate through the app that there is a problem with the power source?