Hi Christian,
Below link shows one of trick to overcome checked "show friendly error message" option
http://blogs.msdn.com/b/ieinternals/archive/2010/08/19/http-error-pages-in-internet-explorer.aspx
"Of course, it is impractical (and not necessarily desirable) to reconfigure IE on all client machines, so most servers that wish to ensure that users will see a given HTTP error message will pad the response with spaces or other characters until the response body length exceeds 512 bytes"
Regards