To avoid confusion this issue is related to accessing the Web UI over a local LAN. When I posted this in troubleshooting there was confusion with people thinking I was talking about accessing it over the internet.
Devices on a local LAN have a device name/computer name. These names can be used to access services on the device. Examples might be file browsing, or a web server (e.g. http://TheServer), or a subsonic music server (e.g. http://TheServer:4040/index.view), or an Emby media server (http://TheServer:8096/web/dashboard.html), and until a recent update , uTorrent (e.g. http://TheServer:9091/gui/).
Now uTorrent no longer responds when using the computer name, giving a 400 error. Using the IP address works fine as before (e.g. http://192.168.0.51:9091/gui/). If I'm actually on the server localhost works and so does the loopback address 127.0.0.1, but not the device name, locally or over the LAN.
This has previously been working for a few years. All other services (as listed above) continue to work with the device name just fine.
This isn't a huge deal but I would prefer to be able to use the device name rather than IP address if possible.
Edited to add: I noticed in the change log for the last release "– Sanity check Host header on HTTP requests" among other changes to the Web UI . Wondering if it is related to this.