Question Problem "Copying assembly from 'Temp?UnityEngine.UI.dll" to "Library?UnityEngine.UI.DLL' falled.

Hello, I'm hopeing some one can help me. I'm new to Unity and am getting this error.
"Copying assembly from 'Temp?UnityEngine.UI.dll" to "Library?UnityEngine.UI.DLL' falled.
I'm working on this project with a fiend and we are using the cloud.

Every project i open or every project i try to create now has this error and will not let me do play test anything. i can not seem to rid my self of this error.. this has happened to me before and resulted in a total format of my computer. I'm rely hoping some one can help me figure this out..

Thank you,

Nathan