Same problem/error message here.
In NGUI/Scripts/Editor there is a script called UISliderInspector. It only includes the following code:
using UnityEngine;
using UnityEditor;
[CanEditMultipleObjects]
[CustomEditor
(typeof(UISlider
))] public class UISliderEditor : UIProgressBarEditor
{
}
Just tried reimporting, no change. Also, there is no other file called UISliderEditor anywhere. Just UISlider, UISliderColors and UISliderInspector.
The error also occurs when I try to open the example scenes with sliders. They work however, but can not be modified as the editor still simply states that multi-object editing is not supported.
Edit:
I was already using a UISlider before. Current version is 3.08, not sure which Version I had before. Should have been 3.x already, probably last updated early November.