The problem is it's trying to do a download and, as you've stated before, you can't access the webpage. I gave you a couple of potential causes so it's up to you if you want to pursue them or some other cause. That said, I downloaded the Windows version and when it starts into the installation (it's not doing a download or if it did it was very quick), it says "this program cannot display the webpage" and says to check the internet connection, retype the address, etc. However, it completes the installation and tries to do a check which won't work but the installation is complete. However, starting the program is taking forever. I wonder if waiting about fifteen minutes after you get that message if it will continue to install.
UniConverter never would start. I decided to uninstall it and, even though it said it was uninstalled, it kept doing something and locked up my computer for the most part. Rebooted and it's gone.