White images with material "None" rendered green

Hi,

I have an issue with my project where new UI images are rendered green when their material is set to None. Even in a new scene, an empty image is rendered green.

View attachment 218160

However, in a new project everything is fine:

View attachment 218161

This old thread sounds similar https://forum.unity3d.com/threads/unity-5-0-material-bug-green-glow.323518/
I'm using Unity 5.5.1f1. Do you have any idea how to fix this?

Thanks,
Alex