I have seen this in the past. Now the fact that you can't even access it through the browser makes it even clearer.
You have some firewall or proxy rules preventing you from accessing the URL.
I suggest getting in touch with your internal IT Team.
Sure .. but I need to have more information before I talk to my network team; i.e. what ports need to be opened. I can't just go to them and do trial and error - it's a security risk. There has to be some documentation on how the traffic flows between Tableau desktop and the connector.