Such an error message is typical for the Internet Explorer browser and it is usually about the object that should have the property, and for some reason that object is null or undefined. This means that in your case for some reason the rotator
variable is null or undefined, not the property autoIntervalID.
The reason for such a behavior may be different and the first that I can think of is reloading the RadRotator through Ajax requests and not refreshing the rotator reference. Still I will need a sample runnable project in order to investigate the issue in details. For that I advise you to open a support ticket and attach the sample project there.
Hope this helps.
the Telerik team
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