I got this annoying error recently in a website I've been working on. It worked perfectly fine locally, it worked fine after uploading to the web server on my machine and other machines in the lab (various browsers). Then, it turns out, it doesn't work on (one of) the client's machines. Of course, I'm unable to replicate the error locally.
I looked around and it seems the first solution was to clear the cache. A lot of people said that Ctrl+F5 works, others said they needed to manually clear the cache and then restart the browser. Well, neither of those worked.
Fortunatelly, the solution turned out to be pretty easy: Set the "CombineScripts" property of the ToolkitScriptManager to false.
<ajaxToolkit:ToolkitScriptManager runat="server" ID="ajaxScriptManager" EnablePartialRendering="true" CombineScripts="false" />