Sign-in stuck on my laptop - works perfectly on PC

Have downloaded a bunch of code updates to my laptop,which is currently connected to the same router as my PC (so minimal difference in IP address) and when I try to signin my app to Dropbox it is just stuck there (as I write this it's been more than 15 minutes, on my 2nd try). I confirmed still works on PC - signed in, signed out, signed in again in under a minute, so no prob with user/password, nor the Dropbox server, and yet the laptop one is just hanging. 

Is there something equivalent to "clear cookies" that I need to do, because I can't see anything to click on that would facilitate such a thing.
This is in C#/.NET. This is where I'm stuck...

void WebViewOnNavigating(object Sender,WebNavigatingEventArgs e)
{
DebugUtil.ConsoleWrite(debugOn,$"{nameof(WebViewOnNavigating)} has started");
if (!e.Url.StartsWith(RedirectUri,StringComparison.OrdinalIgnoreCase)) {
DebugUtil.ConsoleWrite(debugOn,"StringComparison failed");
return;
}
else {
DebugUtil.ConsoleWrite(debugOn,"StringComparison success");
};
DebugUtil.ConsoleWrite(debugOn,"about to start try statement");
try {
DebugUtil.ConsoleWrite(debugOn,"awaiting response");
OAuth2Response result=DropboxOAuth2Helper.ParseTokenFragment(new Uri(e.Url));
// etc.

The debug statements are indicating it's stuck at the "If (!e.Url....." statement. On PC it fails a couple of times and then succeeds. On laptop it fails a couple of times and nothing else happens. The redirecturi I'm using is https://localhost:52475/authorise. I tried http as well - both work on PC, both getting stuck on laptop. Do I maybe need to use a different port on the laptop? I'm not sure how to check that (I used that one to begin with as it was mentioned somewhere here as the one you guys use).

P.S. this is on Windows 10. I just thought to check the versions and my PC is on 1903 but my laptop is on 1909 (I don't know why my laptop is ahead of my PC) - is there maybe a signin issue from 1909? Oh, and also my PC is Pro but laptop is Home.

thanks,

  Donald.