Why is the unity5 skybox yellow when I assign the default skybox?

When I first made my current project I chose 2D as the default settings. Now I made a new scene which I want to be 3D, but when I assigned the default skybox in the lighting window it became discoloured with an yellow colour (see the picture below). I tried adding my own procedural sky box and changing the properties in the lighting window but nothing seems to help.

I hope somebody can tell me what I’m missing, because I feel like I’m missing something obvious.

Did you added a Directional Light to your scene?