A bit more information: I setup my parents several years ago on OpenSprinkler. This is an early hardware version that has an Ethernet jack, though at the moment I can’t say how early it is. It was one of the kits that I assembled.
Again, I can access its web UI without a problem, but the iPhone app gives me the same “cannot connect” error. The web UI says it is running firmware version 1.8.3. Similar to my OSPi, no changes have been made on the hardware side, only an iPhone app upgrade.