Today I started experiencing a weird problem with AjaxControlToolkit Calendar Extender control – it would render at the top left corner of the page, and not under the text box control it was bound to.
Fired up Google Developer Tools and saw an error that was being thrown – Sys.ArgumentNullException: Value cannot be null. Parameter name: elements.
Adding ScriptMode=”Release” to asp:ToolkitScriptManager solved the issue. Not sure why release version of the script libraries would be different to debug version (or why debug version causes the problem), but this solved my problem.