Problem with ASP.NET AJAX Tabs

I get the following error intermitantly from the live website I develop (say twice a month). From the stack trace, it seems to be internal to the AJAX control toolkit.

Error Message: Invalid object passed in, member name expected. (1): {"ActiveTabIndex":0,"TabState":[true,true开发者_开发百科,true,true]}

I have found once other person who has got the exact same error http://objectmix.com/software-testing/356582-microsoft-visual-studio-team-edition-software-tester-web-test.html

Its not a big issue but I'm interested to know what is causing it. Detailed stack trace is

Error Message: Invalid object passed in, member name expected. (1): {"ActiveTabIndex":0,"TabState":[true,true,true,true]} Stack Trace: at System.Web.Script.Serialization.JavaScriptObjectDeserializer.DeserializeDictionary(Int32 depth) at System.Web.Script.Serialization.JavaScriptObjectDeserializer.DeserializeInternal(Int32 depth) at System.Web.Script.Serialization.JavaScriptObjectDeserializer.BasicDeserialize(String input, Int32 depthLimit, JavaScriptSerializer serializer) at System.Web.Script.Serialization.JavaScriptSerializer.Deserialize(JavaScriptSerializer serializer, String input, Type type, Int32 depthLimit) at System.Web.Script.Serialization.JavaScriptSerializer.DeserializeObject(String input) at AjaxControlToolkit.TabContainer.LoadClientState(String clientState) in C:\Program Files\Microsoft ASP.NET\AJAX Control Toolkit 3.0\AjaxControlToolkit\Tabs\TabContainer.cs:line 346 at AjaxControlToolkit.ScriptControlBase.LoadPostData(String postDataKey, NameValueCollection postCollection) in C:\Program Files\Microsoft ASP.NET\AJAX Control Toolkit 3.0\AjaxControlToolkit\ExtenderBase\ScriptControlBase.cs:line 314 at AjaxControlToolkit.TabContainer.LoadPostData(String postDataKey, NameValueCollection postCollection) in C:\Program Files\Microsoft ASP.NET\AJAX Control Toolkit 3.0\AjaxControlToolkit\Tabs\TabContainer.cs:line 447 at AjaxControlToolkit.ScriptControlBase.System.Web.UI.IPostBackDataHandler.LoadPostData(String postDataKey, NameValueCollection postCollection) in C:\Program Files\Microsoft ASP.NET\AJAX Control Toolkit 3.0\AjaxControlToolkit\ExtenderBase\ScriptControlBase.cs:line 425 at System.Web.UI.Page.ProcessPostData(NameValueCollection postData, Boolean fBeforeLoad) at System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint)