You did not answer the question I asked. If you would answer it, that would give us a small clue as where to look.
What is the exact message that appears when it "fails"?
All the rest of your description of URLs etc. is not relevant as far as I know.
The fact that you can connect to your QNAP using the TCP plugin is not relevant since TCP and HTTP are not the same.
Talking to Kodi is not the same thing as talking to your QNAP NAS.
I don't think the fact that Kodi is running on your QNAP is really important. If Kodi is running and configured correctly then you should be able to communicate with it regardless of what platform it is running on. For example, I can control Kodi running on my Android tablet and the setup is the same as for a PC.
The best thing you can do to find your mistake is to go through the installation instructions again, step by step and try not to make any assumptions. Watch the video AND read the wiki page completely. A little bit of knowledge can be a dangerous thing. It seems you skipped the part about installing the VoxTools addon for Kodi, for example.
One thing that I have noticed is that your QNAP appears to be serving its own web pages using port 8080. That could be the problem if you set Kodi up to use the same port. When you set up the webserver in Kodi, you should choose a different port to avoid a conflict since two web servers running on the same machine cannot use the same port.
See:
http://voxcommando.com/mediawiki/index.php?title=Installation_Kodi#Web_server_tabYou will need to choose a free port in the Kodi config (8083 is probably OK) and then update the settings in the VC xJson plugin to use the same port.
Note that I am talking about the HTTP port. Do not worry about the port in "Other settings" for now.
In case it is not obvious, Kodi must already be running on your NAS in order to talk to it from VC.