Rebooting the Pi was my first step. In fact I did it a couple of times between tweaking settings. All to no avail.
I did eventually resolve the problem last night. I use a number of TPLink Ethernet-over-Power plugs to get network around the house. One of these had hung. Once I unplugged and reconnected that, everything started working again.
Although I was using thr player on the same Pi as the server, it is synchronized to other players and I think that must have been the issue.