I am also having the problem with the RadNumericUpDown control. I'm still on 2011 Q3 SP1, but it doesn't sound like that matters. It also happens whether I targert Silverlight 4 or 5...simply having Silverlight 5 dev installed seems to break it.
In my case I kind of know the circumstances that cause it....
I have a tree control on the left with different types of elements:
When you select a Parent, the right side controls fill with Parent properties, when you select a child, it fills with child properties.
The Parent and child are completely different objects so the right side has 2 sets of controls in Borders. When you click on a parent we show the parent border and hide the child border and when you click on a child we show the child border and hide the parent border...one set of controls visible at a time.
The app starts with a Parent item selected, so the Child border is hidden
If I then select a Child with a non-zero value being pushed into the UpDown, it crashes.
If the child has all *zero* values, it does not crash...maybe because the default value is zero and does not trigger a change?
As an experiment I set it so that the child border is *never* hidden and it does not crash in either the non-zero or zero case. It has some connection to the visibility state of the control when you set it.
So, my work-around which is still holding up is to force an update of the Child border before its controls are filled with values:
ChildBorder.Visibility = Visibility.Visible
ParentBorder.Visibility = Visibility.Collapsed
' THIS FIXES IT FOR ME
' Fill Child controls.
With the explicit UpdateLayout after making the border visible, I no longer have the exception, which lets me continue working.