We have noticed that FlyTreeView works wrong in MS Ajax UpdatePanel.
We want to use your control in UpdatePanel so we need only postbacks without any internal XMLHttpRequest. All properties of FlyTreeView like PostBackOnClick was set to true.
In the Firefox we use extension FireBug. And we noticed that node click in your control cause not only using of XMLHttpRequest before the postback but also many independent postbacks which are handled by Ajax. So Ajax sends many XMLHttpRequest and only one is processed correctly. After few such attempts Ajax show error message about incorrect ViewState.
As we understood if set next attributes PostBackOnSelect=\"True\" PostBackOnDeselect=\"False\" PostBackOnDropAccept=\"False\" PostBackOnExpand=\"True\" PostBackOnCollapse=\"True\" PostBackOnClick=\"True\"
then node click initiate next postbacks (which are handled by Ajax) one after another: PostBackOnClick, then PostBackOnDeselect, then PostBackOnSelect, then PostBackOnExpand (real consequence isn't important). Without Ajax seems only one postback is generated. It looks like you don't take into account that your postback may not cause real reloading of page...