Sul sito della Microsoft ho trovato questa spiegazione:

This problem occurs when the browser sends a linear white space to the IIS 6.0 Web server, and the linear white space is between the HTTP request line and the start of the HTTP request header. For example, this problem may occur if the browser sends a space character (0x20) that is between the HTTP request line and the start of the HTTP request header. Subsequently, IIS 6.0 interprets the space character that is between the HTTP request line and the start of the HTTP request header as not valid.