Could you please elaborate a bit more on your scenario and how you set up your tooltip ?
For Q3 release we fixed an exception with similar stack trace like yours , is it possible for you to test your scenario with Q3 dlls, it should be fixed ?
Also , there is no need to create a RadToolTip and then use RadToolTipService (RTTS) over it. This will create two nested tooltips because RTTS creates RadToolTip internally. You need to either create RadToolTip or use only RTTS without creating RadToolTip on your own.
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