Your project is hosted in the Mendix Hosting Enviroment and we are using Nginx as a web server. Nginx standard has some standard configuration called Proxy buffering. As you can see at the Nginx documentation.

The proxy buffer size default is probably too low to handle the getmetadata request as stated in ticket 7336. There are two options to fix this.

1. Turn off proxy buffering
2. Increase the proxy buffer size.

Because Internet Explorer 6 can't handle gzip/deflate requests properly the gzip/deflate option has been disabled to disable support gzip/deflate for Internet Explorer 6. Gzip/deflate wraps the application/json content, the size of the content length of the request is decreased. I think the proxy buffer size is high enough to handle this request for gzipped requests but is too low to handle ungzipped requests.

We tried some configuration changes and managed to recreate your situation in other browsers as well. We've done this by turning the gzip/deflate option off which means that this problem isn't Internet Explorer 6 related. The content length of a certain request is too large compared to the proxy buffer size for ungzipped requests but is enough for gzipped requests that are handled by other browsers.

Btw, did you check whether it isn't a security issue? Users that login without any userroles trigger the same error.

1. I suggest making not-using-IE6 an option, as this will not be your last issue with that browser.
2. I suggest filing a bug report.