Hello asdf sdfasd,
In order to initially set the value of the RadSlider control, you should use its Value property in case IsSelectionRangeEnabled=false and SelectionStart, SelectionEnd properties in case IsSelectionRangeEnabled=true.
Regarding the error that you report - I suppose there is a problem with your setup, but I cannot be sure unless I am able to reproduce the error locally. Therefore, could you please post your code that demonstrates the problem here?
the Telerik team