I’m writing this with a certain amount of egg on my face. It turns out that My OpenSprinkler Controller V 1.4X, in service since 2013, is alive and well. The connectivity problem was caused by a failing Trendnet Powerline Carrier Adapter, which went from intermittent to dead over several weeks. It should have been easy to isolate, but in my haste, I may have swapped out the adapter with another with a similar problem, or even worse, I may have put the same one back in. The troubleshooting was further complicated by trying to hook up a laptop, thinking it was just like connecting to one of my IP Cams, and not needing any special drivers or setup that I didn’t provide.
Once inside and connected to my router, as Ray suggested, it was immediately clear that things were working normally. It is back outside operating as before, and will probably again live through the 115+F temps that are coming again soon. The second good news it that I’ll have a backup that I can put in place with minimal effort.