Have you tried taking a look at the logs on your local dev instance when the error occurs? Usually when there's a 500 Jive will log something (though sometimes it's incomprehensible) so you may find that useful.
As you mentioned, the proxy seems to be working for other files, I've just tried it with a file in one of my apps and that seems to work.
Unfortunately, there does not appear to be anything relevant in the log for the local development instance. Of course, there is lots of stuff spewing in the log all the time ... :-( ... but mostly about not being able to contact some 'Single Tenant Heartbeat' service/server, and a warning about checking firewalls (although those errors are spit out even when I'm not logged into the server, so it does not appear to be related to anything that I am doing).
I am seeing the proxy Error 500 from the browser, when I look at the browser console as I load a page containing my app.