DevTrace breaks AJAX with Vagrant & Custom Domains

[jamie]

When DevTrace is enabled in my local environment, I'm getting broken XHR responses in my app.

Environment: Rails 3 on passenger on nginx, inside Vagrant/VirtualBox. We're using custom subdomains for namespacing, but I'm not actually sure if that's relevant, other than I'm not directly visiting localhost:3000 in development.

Behaviour without dev_trace - XHR request returns a 200 OK with expected response

Behavior with dev_trace: true - XHR request returns a 502 Bad Gateway, but backend rails logs still show successful response.

In both cases, the browser was sending identical requests & headers, and if I copy the request as a curl command, I can reproduce the differing behaviour just by restarting the server in vagrant with SCOUT_DEV_TRACE=true.

I'm not sure what other info will be helpful in tracing this, but do ping me (I can repro trivially, and get whatever you need).