There are no connection lights, despite switching cables with another device that is connecting properly (and continues to work properly with the cable that I was using for the ISY).
It's definitely not an IP conflict, because there is no connection in the first place, and also because I don't have any devices not using DHCP, including the ISY.
I haven't tried the factory reset, but it seems like it wouldn't accomplish anything since it's only advised for conflicts with a fixed IP that can't be resolved.
Unless there's something else to try, this looks like an issue that will require an RMA.