I was not able to reproduce the exact reported error, but there are some improper configurations in the provided code snippet that may lead to such errors:
- The RadSlidingPane, RadPane, and RadSplitBar controls cannot be updated directly with an Ajax Manager control - AJAX updates.
- By design RadSlidingPane (and RadControls at all) supports only ClientIdMode="Auto" and it is highly recommended to configure it this way in order to generate correct client IDs. When ClientIdMode property is set to static many unexpected errors could be experienced.
I hope this information will be helpful.
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