The network connection succeeds but your Denon doesn't say anything at all. There's a bug in the plugin where it doesn't recognize this as an obvious problem and just assumes the default (single zone, standby). Ten minutes later, because you checked the keep alive checkbox, the plugin finally realizes that it's heard nothing from the Denon, gives you an error message, and tries to reset the connection. That doesn't help.
I'll fix the initialization bug, after which you'll get an immediate error when you turn on the plugin. But the actual problem is that your receiver sends no data - or else, your serial interface box drops it. I can't tell which one it is. Please double-check that your serial port settings on the network box are those expected by your Denon; that's the most likely problem.
Perform a processor reset on your receiver. Sometimes the Denons just go into a funk and stop talking.
If you have a spare USB serial adapter, try to use it (instead of the Startec). If your connection is good (and your Denon isn't funked out), then just frobbing the volume control should produce output no matter what.
That's pretty much all I can do to help here. Let me know how it goes.
Cheers
-- perry