Your code seems to be correct and the only thing I can assume that might be causing the problem you report is that you have ajaxified the RadCalendar and the RadToolTipManager. If so, note that the RadToolTipManager uses internally AJAX if used with LOAD functionality and thus you probably get nested update panels as a result.
To test my assumption, please remove all the AJAX settings from your page and check whether the issue persists. If it disappears, the assumption is correct and what you should do is to replace the AJAX settings with standard update panels with UpdateMode="Conditional"
- this will prevent the outermost update panel from update when the tooltip is being shown. More information about nested update panels is available below:
In case my assumption is not correct or you need further assistance, please prepare a sample, fully runnable reproduction demo, open a new support ticket and send it to me along with detailed explanations of the reproduction steps and I will do my best to help.
All the best,
the Telerik team
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