Could you please tell me what browser you are using? We had one similar case when the user was getting such error with the XmlHttpPanel control, but it was only under IE6, and the problem was related to the browser itself. Here is a detailed explanation:
The main problem comes from the fact that Internet Explorer 6 does not support gzip compression, and all the other major (modern) browsers (IE7 & 8, Firefox, Opera, Safari and Chrome) support.
Let me explain in details how the problem is caused.
If the cached scripts are non-compressed, the modern browsers will request for compressed scripts, and this again will cause 4 scripts to be loaded.
All this being said, the issue is related to IE6 browser's support for non-compressed scripts only and not to our control. I think Microsoft introduced support for compressed scripts in the latest updates for IE6. Please download the latest version of IE6 and see if the problem still occurs.
If the problem persists please send us some sample code so we can debug it locally and provide an adequate solution.
the Telerik team