[BUG] Linear Color Space now affects UI ?

My project always was in Linear Color Space. I updated from 5.0.1 to 5.1 today and then my UI was rendered very dark. On changing the color space to Gamma, everything was rendered fine again.

This appears to be a bug, as it worked like a charm before. The strange thing is: after additively loading a scene, the buggy UI in Linear color space suddenly renders correctly and is not dark anymore.

Has anyone else encountered this problem?

I've attached 2 images, showing the buggy dark rendering linear space in comparison to the scene view and the
correct rendering gamme color space, also in comparison to the scene view.

 

Hello, any news about how did you solve it? I am experiencing this when switchin from unity 2017 to the latest 2018.
I will appreciate any insight!