Just one thing to clarify: in Indigo the reverse proxy is provided by a "System Stack nginx" service, so in your setup there will be two Nginx instances. It looks like you've been looking at the logs for the System Stack's nginx; be sure to check the logs for your stack's nginx as well. I plan to eventually provide easier access to the service logs, but for now you'll have to right-click the nginx service in your stack, and choose "Show Compiled in Finder..." from where you can find the relevant logs (and conf) directory.
A few ideas...
Have you checked your PHP logs? Again, right-click your PHP service — you'll find its logs in the Finder there.
It might be worth trying to access your site directly, bypassing the reverse proxy, to confirm the problem is there; do this by appening its port, which you can find in the site configuration. eg for the below config, you'd visit https://drupal9.test:50004.
As you can see, I just installed D9 inside Indigo hoping to replicate the issue you've raised but it installed and ran perfectly; I'm guessing this issue is caused by one or more contrib modules, and the basic Umami Foods demo site is too lightweight. If you think you might know how I could replicate it, I'd be intrigued. However I am hopeful we can get your site up and running without having to replicate the issue myself.
fmitchell If I manually copy the .indigo/stacks/system-stack/nginx_reverse_proxy/conf/sites-enabled/my-site-conf. to the stack location with the nginx_B231 name, it doesn't build and says it cant find the host.
Aha, yes, with the above you'll be overwriting your stack's nginx config with the system stack's reverse-proxy nginx config. The config for your nginx will be at something like ~/.indigo/stacks/STACKID/nginx_B231/conf
The config suggested in that Stack Overflow solution (ie as per below) is in fact already in the standard Indigo nginx config for a site. Perhaps it's worth trying it outside the location block?
fastcgi_buffers 16 16k;
fastcgi_buffer_size 32k;
