The error is produced, because of mismatch of the controls/holders in the template and their parent ids. The error is thrown when the parent of one control/holder do not match correct child and docks cannot be arranged properly. Could you give us some steps( or send a screencast) that we could follow to replicate the issue here. After the page and controls have been added have you made some changes to the ContentPlaceHolders, layouts, removing and replacing templates etc?
The issue could happen in cases when you have removed some controls - physically deleted. When the page is loaded, it is divided on holders and the error says that when an element cannot get the siblings of matched elements.
All the best,
the Telerik team
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