'a._xmlHttpRequest.readyState' is null or not an object

Hello everybody 
We are using the ASP.Net AJAX Framework for http requests to our webservices.
Recently I got this error message out of the blue (Web application running and a window popping up): 
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Microsoft JScript runtime error: 'a._xmlHttpRequest.readyState' is null or not an object 
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When I debug with Visual Studio and draw object a to the watch list I see "a" is an object but with "a._xmlHttpRequest" i get :

a._xmlHttpRequest      -->  Variable uses an Automation type not supported in JScript              Error


and with "a._xmlHttpRequest.readyState"

a._xmlHttpRequest.readyState   -->  a._xmlHttpRequest.readyState' is null or not an object                  Error


This is th ...