The problem you report is most probably caused by a server error in your code and it is shown as an alert due to the fact that AJAX is used. What I can suggest is to carefully debug the server code and see what causes the problem.
Note, also that MS AJAX does not allow to use some server methods as e.g Response.Redirect, Response.Write, etc. - you can see what are the methods which are supported in postback scenarios but not in AJAX ones below:
Since RadToolTipManager uses MS AJAX internally, in case you are using some of the above methods in the OnAjaxUpdate event, this is the reason for the issue. Please, check this out and revise your logic if this is the case.
In case you need further assistance, please prepare and send me a simple, fully runnable reproduction demo along with detailed reproduction steps and I will debug it for you and I will do my best to help.
All the best,
the Telerik team
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