i rebuilt my grid in a new solution, but it also failed.
Here i uploaded it for you:
I want to apply a tooltip with the RadToolTipManager to the whole Griditem and not only one GridData
Is it possible that it's bugged using it on a GridItem instead of a GridData
Did you try to apply a tooltip to a GridItem?
I'm not using Response.Write() anywhere as u see in my test solution.