I can call the webservie directly to the browser with the following URL and it returns be all what I want :

http://localhost:64438/MySearchAutoComplete.asmx/GetCompletionList

When I add it to an autocompleteexetender into the Default.aspx page like that :

<cc1:AutoCompleteExtender ID="AutoCompleteExtender1" 
                  TargetControlID="TextBox1" 
                  runat="server" 
                  ServiceMethod="GetCompletionList" 
                  ServicePath="http://localhost:64438/MySearchAutoComplete.asmx" 
                  CompletionSetCount="12"
                  MinimumPrefixLength="1" />

The page load, I have a textbox but I have an error 500 every time I add a keystroke in the textbox. I see the error in the FireFox FireBug.

http://localhost:62702/   --->This is the webpage that load fine 

--> This is the error

Any idea? I have noticed that I need to attach the process to debug the webservice, I might do something wrong with it too?