It does sound to me, like a permission problem via either a firewall or otherwise.  Make sure Melodyne is not working when you try to register it, that might sound obvious. 
 
Technically speaking, you have already been licensed by the purchase of Sonar Platinum. I would try to reinstall it from the control center. Have you contacted either Cakewalk or Melodyne about the problem? They may prove more useful. You are only marrying up the details on the Celemony website.
 
For AD2 I went to my cakewalk personal product page and downloaded the XLN installer from their webpage. From there, I registered my code, selected 3 libraries and launched the installler. Granted it took a while to work this out however it does work with perseverance.
 
It seems to me that some of the 3rd party plug-ins do you require external registration to fulfill their contract. Also to encourage sales of expansion packs or to update the offered software to a more featured version.
 
 
 

Thanks for the message.  In response to your question as to whether I'd been in touch with the vendor, like mentioned upstream a ways, Celemony technical support told me (surprisingly quickly in my point of view) that they were out of ideas.
 
I'm curious about what you mean about 'Make sure Melodyne is not working when you try to register it, that might sound obvious.'
 
- I have never seen the registration assistant window so how else would I see it except by trying to open Melodyne either by a Region Fx or by inserting Singletrack as a Rewire device? In which case, Melodyne would be technically working (although in this greyed-out Playback mode I'm stuck in).
 
And also for what it's worth I don't think it's a firewall issue. I've turned the net off and tried. I've tried in Safe Mode. I've added DEP exceptions.