Thanks, Bert. I think I figured it out. It seems that something underlying ACv8.3 (perhaps one of its installed dependencies) is of a vintage that doesn't play nicely with recent versions of Win10. It would have to be in the networking components, as the AC app itself runs fine on Win10. But bring it online with other computers in the network, and all hell breaks loose. I went back to using another "main" computer running AC 8.3 on Win7, and everything is fine once more; the networked computers are even more likely to show up in the "networked devices" panel of the other computers,i.e. those which are currently running Win10.
So, rather than stir the hornets nest, I'll simply make sure I always have at least 2 good PCs with 8 or 16GB RAM that can run Win7.