I had a perfectly working ui, but suddenly, on day, Unity decided to break it.
It changed the anchor presets and text fonts. In addition, it changed the scales, the paddings and the positions of some ui objects to NaN.
I have no idea what is going on, and I will have to change ALL the settings back 
Since I literally haven’t touched anything, this really looks like a Unity bug.
In addition, the time it broke, the scene, the game and the hierarchy tabs where blank when I first started Unity, so I had to change my layout to refresh them (by going to Window > Layouts > 2 by 3 at first, then Window > Layouts > Default)
Does anyone knows how to avoid this ?