I already answered your other thread and for your convenience and for others who might encounter the same problem, I pasted my reply below:
The only thing I can assume without having your code is that you have missed to declare a ScriptManager before teh tooltip is added to the page. If so, please note that script controls should be registered in the ScriptManager and this is the most probable reason for this error. Another option is that you are adding the tooltip in some event which is not suitable considering the page lifecycle but I cannot tell you the exact reason until I am able to see what is actually goind on.
This being said, if the problem persists, please provide code on base of which a fully runnable reproduction page could be created and I will do my best to help.
the Telerik team