Debug

While using the System.Net.WebClient you might get an HTTP-Error 400 - Bad Request, unfortunately no further information is provided by the Exception-object, what particularly went wrong.
First check, that you send the correct URL (test with your webbrowser - it should work), assure, that you send the Accept-GZIP-Encoding and recheck your firewall settings, if that all failes, try the following:

Enable .NET logging

Create or open the App.config of your (executable) application and add the following code. On building your solution it will be renamed to {applicationname}.config and placed in the output directory automatically.

<configuration>
   [...]
   <system.diagnostics>
      <sources>
         <source name="System.Net">
            <listeners>
               <add name="System.Net/>
            </listeners>
         </source>
         <source name="System.Net.Sockets">
            <listeners>
               <add name="System.Net/>
            </listeners>
         </source>
         <source name="System.Net.Cache">
            <listeners>
               <add name="System.Net/>
            </listeners>
         </source>
      </sources>
      <switches>
         
         <add name="System.Net" value="Error"/>
         <add name="System.Net.Sockets" value="Error"/>
         <add name="System.Net.Cache" value="Verbose"/>
      </switches>
      <sharedListeners>
         <add name="System.Net"
                 type="System.Diagnostics.TextWriterTraceListener"
                 
                 initializeData="my.log"/>
      </sharedListeners>
      <trace autoflush="true"/>
   </system.diagnostics>
</configuration>

Now start your application and have a look into the written logfile, I mostly got "Client does not support gzip encoding" caused either by firewall or by proxy.