Intermittent NotSupportedException using AjaxControlToolkit version 3.5.60919 (.NET 3.5)

We have an ASP.NET web application using .NET 3.5 and version 3.5.60919 of the Ajax Control Toolkit and we intermittently have been getting the following exception since we started using that version:

Source:          AjaxControlToolkit

Module Name:     System.Collections.Generic.List`1[AjaxControlToolkit.ToolkitScriptManager+ScriptEntry] DeserializeScriptEntries(System.String, Boolean)

Exception Type:  System.NotSupportedException

Error Message:

Assembly "AjaxControlToolkit, Version=3.5.60919.0, Culture=neutral, PublicKeyToken=28f01b0e84b6d53e" does not contain a script with hash code "5546a2b".

Stack Trace:

   at AjaxControlToolkit.ToolkitScriptManager.DeserializeScriptEntries(String serializedScriptEntries, Boolean loaded) in c:\TeamCity\buildAgent\work\86982cda3515e83b\Server\AjaxControlToolkit\ToolkitScriptManager\ToolkitScriptManager.cs:line 633

   at AjaxControlToolkit.ToolkitScriptManager.OnLoad(EventArgs e) in c:\TeamCity\buildAgent\work\86982cda3515e83b\Server\AjaxControlToolkit\ToolkitScriptManager\ToolkitScriptManager.cs:line 186

   at System.Web.UI.Control.LoadRecursive()

   at System.Web.UI.Control.LoadRecursive()

   at System.Web.UI.Control.LoadRecursive()

   at System.Web.UI.Control.LoadRecursive()

   at System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint)

We were using a previous release but I switched to this version so I could use the AlwaysVisibleControlExtender, which was not part of the previous release we were using (version 3.0.202229).

Upgrading the site to a higher .NET version isn't an option.  Version 3.5.60919 of the AjaxControlToolkit is the latest release and I am assuming that there will be no more releases as 3.5 is no longer the current .NET version.

Any ideas how I can resolve this problem?