Hard to say what might be causing this, but there is a good chance (judging by the browser response) that DNS resolution somehow breaks down on the Win 10 PC.
I *think* that it happened once without RU being open at all. I don't think that RU is the cause of the problem here, BUT it increases the frequency of the problem from days to minutes!
I think that Viewer cannot connect because
of a DNS issue on that computer, and not vice versa :) Note, that Viewer and Host also require DNS resolution to work properly if you use the Internet-ID connection type.
In other words you should figure out why DNS resolution fails after some time on that computer. Even if running a Viewer somehow triggers that process, there must be a root cause for that. If you use your ISP's (default) DNS servers you can change them to Google's
, or any other publicly available and proven DNS servers.
Hope that helps.