I had the same issue when I was trying to connect to my IIS .NET Webservice from the Android emulator.
npm install -g iisexpress-proxy
iisexpress-proxy 53990 to 9000
to proxy IIS express port to 9000 and access port 9000 from emulator like "http://10.0.2.2:9000"
the reason seems to be by default, IIS Express doesn't allow connections from network https://forums.asp.net/t/2125232.aspx?Bad+Request+Invalid+Hostname+when+accessing+localhost+Web+API+or+Web+App+from+across+LAN