The first thing you need to do to overcome this error is to check the Proxy Settings . On the left menu, click the three dots then scroll down a little and click on Proxy Settings. The client uses Internet Explorer's settings which means you have to click the Lan button to continue. If you have a proxy set up, disable it, restart the SI Client and then check for updates again. If you keep receiving the same error, completely remove the client and then install it again from its page.
Of course, there is the possibility the server is in maintenance mode in which case you should try a bit later. I have the client installed and it's working as it should.