ESET is not too crazy, I think I use that one on my work machine and it’s never caused me a major issue. The ones mentioned are generally problematic, which is why I asked.
You may want to ensure there are firewall exceptions in place for SketchUp as best practice anyway.
I’m just wracking my brains for other things to test and try.
Which browser are you using?
I’ve (and with your advice / tips / suggestions) finally solved it - It was blinkin ESET…
I just went to network protection and disabled ‘firewall’ and ‘network attack protection’ and ‘botnet protection’, and then launched the sign in, and it went straight through, I then enabled the three again !
I will mark this as a solution - just so we all know what the culprit is ! I suppose Eset has done some update over the last month and changed something ? I suspect I’ll learn how to add this particular app or create a rule or for now I’ll just do this once a month lol!
Sorry for all the aggro and time spent reading this - and thanks again to all that helped out 
To clarify, 127.0.0.1 is special. It connects between apps on the same computer without ever even attempting to reach the internet. So, settings on your router are usually irrelevant.
The message you are getting means that for some reason the local license server that SketchUp runs on your computer is either not started, hung, or being blocked by something else on your computer. It could also mean that it couldn’t “phone home” to Trimble to validate your license. Once it gets that validation, it remembers it for about a month and SketchUp will run without needing the internet.
You should try doing the repair of the installation and then restarting the computer. If you are a Windows guru you could see if the license server has been disabled in system settings. It’s been too long since I switched to Mac that I can’t recall what setting does that, but I’m pretty sure there is one.
Edit: you subsequently found that indeed the connection home was being blocked. The message about 127.0.0.1 was a bit of a red herring then, because it misled as to the actual problem.