...don't understand, why Android with mobile hotspot allows to start Client and server, while connected Android only allows to start server, even if server is started on mobile hotspot Android ?
... now I exchanged the mobile hotspot direction and have different result. Button 'Connect at client' is enabled now, but while connecting, there is a android.system.ErrnoException: Connection refused. So it seems to be a handy specific error. I'm gone to try further