I'm receiving the error in the title occasionally when working with FlyTreeView for ASP.NET.
The stack trace is:

Specified cast is not valid.
   at NineRays.WebControls.FlyTreeView.ApplyStateLog(String stateLog)
   at NineRays.WebControls.FlyTreeView.System.Web.UI.IPostBackDataHandler.LoadPostData(String postDataKey, NameValueCollection postCollection)
   at System.Web.UI.Page.ProcessPostData(NameValueCollection postData, Boolean fBeforeLoad)
   at System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint)

when populating the tree with some nodes based on a selection.

The software that uses the FlyTreeView is used by several users and the error above appears only to some users.
What can be the reason.
The tree is populated always in the same way, so I don't understand why some users get the errors and others not.

The version of DLL is

Thank you very much.