AjaxControlToolkit error, Collection was modified; enumeration operation may not execute

We have a production web site that we recently upgraded to the July 2013 release of the Ajax Control Toolkit. Since then, we're seeing an unhandled exception in the application event log. There doesn't seem to be a pattern to the error, and we only see it about once a day even though we have hundreds of active users. We don't believe we've made any other changes that are related to this problem. It doesn't seem to be related to any other events, like recycling or time of day.

Is anyone else seeing this error?

Any idea how to troubleshoot this one?

Thanks for any ideas!

Event ID: 1309

Event code: 3005

Event message: An unhandled exception has occurred.

Exception type: InvalidOperationException

Exception message: Collection was modified; enumeration operation may not execute.

The full text of the event is shown below:

Source: ASP.NET 4.0.30319.0
Event ID: 1309

Event code: 3005
Event message: An unhandled exception has occurred.
Event time: 8/27/2013 4:25:11 PM
Event time (UTC): 8/27/2013 8:25:11 PM
Event ID: b471019e51c94fc8b835f20803b11d23
Event sequence: 8523
Event occurrence: 19
Event detail code: 0
 
Application information:
    Application domain: /LM/W3SVC/2/ROOT-1-130221037256083188
    Trust level: Full
    Application Virtual Path: /
    Application Path: C:\Users\Administrator\Documents\AdaCareWeb\
    Machine name: H-ADACARE
 
Process information:
    Process ID: 5784
    Process name: w3wp.exe
    Account name: NT AUTHORITY\NETWORK SERVICE
 
Exception information:
    Exception type: InvalidOperationException
    Exception message: Collection was modified; enumeration operation may not execute.
   at System.Collections.Generic.List`1.Enumerator.MoveNextRare()
   at System.Linq.Enumerable.All[TSource](IEnumerable`1 source, Func`2 predicate)
   at AjaxControlToolkit.ToolkitScriptManagerCombiner.GetScriptReferences(HttpContextBase context, String[] bundles)
   at AjaxControlToolkit.ToolkitScriptManagerCombiner.GetCombinedScriptContent(HttpContextBase context, String[] bundles)
   at AjaxControlToolkit.ToolkitScriptManagerCombiner.GetCombinedScriptContentHash(HttpContextBase context, String[] bundles)
   at AjaxControlToolkit.ToolkitScriptManager.OnLoad(EventArgs e)
   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.Control.LoadRecursive()
   at System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint)

 
 
Request information:
    Request URL: https://my.adacare.com:443/Login.aspx?ReturnUrl=/
    Request path: /Login.aspx
    User host address: 96.10.192.232
    User:
    Is authenticated: False
    Authentication Type:
    Thread account name: NT AUTHORITY\NETWORK SERVICE
 
Thread information:
    Thread ID: 40
    Thread account name: NT AUTHORITY\NETWORK SERVICE
    Is impersonating: False
    Stack trace:    at System.Collections.Generic.List`1.Enumerator.MoveNextRare()
   at System.Linq.Enumerable.All[TSource](IEnumerable`1 source, Func`2 predicate)
   at AjaxControlToolkit.ToolkitScriptManagerCombiner.GetScriptReferences(HttpContextBase context, String[] bundles)
   at AjaxControlToolkit.ToolkitScriptManagerCombiner.GetCombinedScriptContent(HttpContextBase context, String[] bundles)
   at AjaxControlToolkit.ToolkitScriptManagerCombiner.GetCombinedScriptContentHash(HttpContextBase context, String[] bundles)
   at AjaxControlToolkit.ToolkitScriptManager.OnLoad(EventArgs e)
   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.Control.LoadRecursive()
   at System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint)
 
 
Custom event details: