If you happen to have a laptop with a serial connection, then a serial cable will work - no USB-to-serial adapter needed.
Let me make one small correction. The Marantz and Denon devices use a serial connection. If you have a serial (RS-232) connector on your computer, you make a direct serial connection. If, otoh, your computer lacks a serial connector, you will have to use a serial-to-USB adaptor in order to make this work.
OK, let's go through the start-up sequence in a little more detail. I have a 4520 as well, so yours should be working the same as mine.
1. You are using Pro version 3.6?
2. When you launch the software, you are choosing the 4520?
3. You get a pop-up box asking which USB port to use?
4. After you select the USB port, the TV displays "MultEQ Pro Calibration"?
5. Does the license key pop-up ever appear?
BTW, I never hear any relays clicking on my 4520. Not sure why the relays would click at the beginning of the software launch.
In Device Manager, does the Prolific interface look like this?
I was not successful testing on Win8. My Win8 installation is the 8.1 preview edition. In order to install the Pro software, I need to install the .NET Framework Ver 2. Several attempts to download and install this software failed, so I have abandoned the effort. Perhaps another Pro user is successfully using Win8 and can comment.
Otherwise, my recommendation would be to contact Audyssey installer tech support, or Denon support.
I agree that we should add some recommendations for those who experience the issue. But I don't think we really understand what is going on yet. I am using all four network connections on my 4520, and I have never experienced the issues you faced.
Does anyone have an idea what might be the root cause of this problem?