The only thing I can assume that might be happening without examining your sample code is the following:
This issue occurs in a scenario where you have a tooltip manager inside an ItemTemplate of a grid and this means that an update panel is actually there. When you rebind the grid, it clears its Controls collection and rebuilds the controls again - however, there is a known problem with the ScriptManager control which holds a reference to the old, already not existing update panel and this causes the issue. The problem can be reproduced in this very same scenario without using RadControls at all but only standard ASP.NET controls.
What I can suggest in order to fix the problem is to put the RadToolTipManager outside the item template - in this manner you will be able to add targets to the TargetControls collection in the OnItemDataBound event of the grid . I hope that my explanations and the suggested solution helps, let me know how it goes.
the Telerik team
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