requests from Virtualbox gets cut off when visiting websites running on MacOS Sierra

Question

The situation: I'm running the following things on my MBP with macOS Sierra:

• A python SimpleHTTPserver serving a Django app on localhost:8000.
• A VirtualBox (5.1.8) image running with Windows 7 (downloaded from modern.ie)

I started IE11 inside this Virtualbox and visited http://10.0.2.2:8000 (as it should be according to the docs). I noticed that only a small part of the content was received, it looked like the content was cut off after a certain amount of bytes.

I started Chrome to check if it's a IE11 specific problem, but the same problem occured. When I ran Fiddler to sniff the requests, the following message showed up in both IE11 and Chrome:

[Fiddler] ReadResponse() failed: The server did not return a complete response for this request. Server returned 25,560 bytes.

The amount of bytes that is shown in the message above changes every request, varying between ~17 bytes and ~30,000 bytes.

When I looked in Fiddler at the requests made I noticed that the response body is always 512 bytes, and that the HTTP status code is 504.

I asked a colleague to run the same application on his MBP, with python manage.py runserver 0.0.0.0:8000 so that I'm able to browse to the webserver he is running. When I visited http://[his ip]:8000 I sometimes get a 200, but usually a 504 with only the first 512 bytes as response body.

This problem doesn't occur at all when visiting other websites (like google or stackoverflow). However this problem does occur when running other Django apps.

Is there anything wrong with my Virtualbox configuration? Or is it a Window 7 configuration issue? How do I fix this so that these requests don't get cut off?

Edit 1: This issue could be caused by something in macOS Sierra, because one of my colleagues with OSX Mavericks does not have this problem, while another colleague with macOS Sierra has the same problem as I have. What could this be?