This is not related to the Html Agility Pack directly, but rather to the underlying HTTP/socket layer. This error means the server is not sending back a correct HTTP status line.

The status line is defined in HTTP RFC available here: http://www.w3.org/Protocols/rfc2616/rfc2616-sec6.html

I quote:

The first line of a Response message is the Status-Line, consisting of the protocol version followed by a numeric status code and its associated textual phrase, with each element separated by SP characters. No CR or LF is allowed except in the final CRLF sequence.

   Status-Line = HTTP-Version SP Status-Code SP Reason-Phrase CRLF

You can add socket traces with full hex report to check this:

<configuration>
    <system.diagnostics>
        <sources>
            <source name="System.Net.Sockets" tracemode="includehex">
                <listeners>
                    <add name="System.Net.Sockets" type="System.Diagnostics.TextWriterTraceListener" initializeData="SocketTrace.log" />
                </listeners>
            </source>
        </sources>
        <switches>
            <add name="System.Net.Sockets" value="Verbose"/>
        </switches>
        <trace autoflush="true" />
    </system.diagnostics>
</configuration>

This will create a SocketTrace.log file in the current executing directory. Have a look in there, the protocol violation should be visible. You can post it here if it's not too big :-)

Unfortunately, if you don't own the server, there is not much you can do (if you already added the useUnsafeHeaderParsing setting, which is good) but fail gracefully in these cases.