My camera goes blue when running Unity3D + Vuforia

I am newbie at Unity3D and Vuforia.

Unfortunately, sometimes when I run my application the screen of my camera goes blue as you can see in this picture

My project is composed by a directional light, an AR camera and an Image Target.

I can't figure out what's wrong because this problem happens only sometimes and often when I resume the activity. Can you help me?