We are aware of a problem with the DockedPaneID and ExpandedPaneID properties when they are set and the RadSplitter is an INaming Container as a MasterPage. If you have used those properties, the issue is already fixed in the SP which is scheduled to be release today or tommorrow and once you upgrade to it, everything should start working as expected.
If this is not the case, please prepare some reproducetion code and detailed explanations of the exact problem and a screenhot. Once I receive them, I will create a demo based on your code and explanations, I will examine it locally and help you solve the issue.
the Telerik team