Hmm, this is very strange and frustrating. It feels rather like https://forums.indigostack.app/d/67-reverse-proxy-not-working-with-mamp-stack/2 which at this point is still unresolved, primarily because I simply cannot replicate it, and because after extensive research neither @crystalenka nor I could ascertain anything unusual at all about her environment.
Your screenshots are most helpful, thanks! They indicate that the stack has indeed started up and thus should be hosting the welcome site on a randomised port. The reverse proxy is then failing to start due to not finding the
Are you able to launch Indigo without it attempting to auto-start "My First Stack"? If so, could you try creating a new stack with only nginx and one site? Give it hard-coded ports (eg 5080 and 5443 or whatever), and see if it starts up. If the stack starts but the RProxy still throws an error, leave the error dialog unanswered, and see if you can load your site in the browser by using its ports eg
https://mysite.test:5443. If that doesn't work, try
One other thought: do you happen to have access to a second Mac, or Mac virtual machine (eg with UTM)? It would be very interesting if you could try it there also.
I would really love to get to the bottom of this issue. My theory at this point is that some Macs are not able to resolve indigo's domains (eg "mysite.test"). The only thoughts I can come up with regarding that are perhaps due to their DNS configuration is interfering (perhaps due to ISP or router settings?) or there's some left-over environmental thing in their Mac from something previously installed (eg
/etc/hosts entries). However, in @crystalenka's case, none of that seems the case...