IIS ARR/Reverse Proxy 502 Errors for Larger Files

I have an API written in Python which I am accessing with IIS 7.5 via the ISAPI extension PyISAPIe. I thought it would be sensible to separate this API from our main site, so it is in a different site accessed with port 8008.

To be able to access the API with URL like http://domain/api, I have created a reverse proxy rule in IIS with ARR and URL Rewrite so that http://domain/api is redirected to http://domain:8008.

The reverse proxy seems working correctly and I am receiving the correct responses from the API in most cases (the API returns JSON). However, in some cases, I am getting 502 errors with the following message:

502 - Web server received an invalid response while acting as a gateway or proxy server.

This seems to be happening only when the JSON response is above a certain size (about 125KB in my tests) – for smaller sizes, everything seems to be working correctly.

I've tried increasing the Response buffer threshold in the proxy settings to no effect.

Addtional Information

In the logs I can see that the requests to the site on port 8008 are succeeding and are returning a 200 status code.

After activating Failed Request Tracing, the only error/warning is the following (this is after recieving the response from the other site – see previous point) :

MODULE_SET_RESPONSE_ERROR_STATUS 
Warning ModuleName="ApplicationRequestRouting",
Notification="EXECUTE_REQUEST_HANDLER", HttpStatus="502", 
HttpReason="Bad Gateway", HttpSubStatus="3",
ErrorCode="2147954552"ConfigExceptionInfo=""