Fabio, I do not know how to troubleshoot your issue any further, but I have a thought… maybe it will help… from that little that I gathered from reading forums, Zwift consists of two parts, zwift launcher which is basically a script which runs in Internet Explorer, and zwift.exe which is the game itself. Zwift launcher is responsible for logging in and for checking for updates. For one thing, if you do not have Internet Explorer on your PC at all (the current version of IE is 11, and it is not the same as Edge), it might interfere with the loader/launcher. Also, there may be some Internet security features which you may have enabled in IE which could prevent scripts from running. Check on that. Maybe reset IE preferences to “factory default”. Also, about a year ago, there was an error caused by a certain script which launcher apparently downloaded from a storage server on Facebook. Many users reported it. That error did not crash Zwift nor did it prevent it from running, but this makes one think that firewall or VPN that blocks certain sites or connections could in theory interfere. I do believe that in you Documents folder you can also find log files of zwift launcher. Maybe they show something of interest, like some connections which did not happen.
Please note that you may have some sites restricted not only on your computer, but also on your home cable or DLS modem, wi-fi router, or VPN. Network provider is less likely, but who knows. As an idea, you could create a wi-fi hot spot on your cell phone, connect your PC to your phone, and see if update goes through with the hotspot. If it does, it means something is restricted on your network that is not restricted on your cell phone internet connection. Or take your computer to a friend, connect to their internet, and try updating from there. This could provide you with a valuable insight into “what it is and what it is not”.
You could also temporarily disable firewall on your computer or router to see if this helps. If it does, the next step will be to troubleshoot what firewall setting causes it. If your installer sits on downloading a benign video file for a while, maybe it simply times out every time because it cannot download one of the assets.