I have tried to install plugin using X adds on also on your setup and it seems that the connection from servers to retrieve the addons list is blocked by your hosting service provider. Please check error log. Let's see if there's anything more substantial that will give us more information on the issue. Then, contact your hosting regarding this issue.
I suggest that you temporarily change your hosting service provider and Move Your Website to a temporary hosting or your local machine to fix the issue, After the installation of the addon you will be able to move the site back to your sever.
Let us know how it goes.