Shadows render scene unusable after update to 5.5

I created a project in 5.4 but at some point, the level I’m working on isn’t loading anymore (UE crashes when world partition map is top be drawn???) so I upgraded to 5.5.Only yesterday, I noted that the lighting in an asset put in a hole in the landscape is all black when directional light doesn’t shine in it directly. I disable ExpronentialHeightfog while editing for practical reasons
.This is the scene in 5.4 (taken before the crashes)


and this the same one un der the same lighting conditions

It gets worse when I get nearer to the asset. This is from a distance:

and this when I have entered the vicinity:

Enaabling ExponentialHeightFog makes it worse, I have to minimize primary and secundary density and falloff, and that shows weird lighting:

I have the impression that something in ExponentialHeightfog has changed because I didn’t change a thing in lighting (except disabeling ExponensialHeightFog when editing, or the directon of DirectionaLight); the 5.4 version wasn’t as light as 5.5 !

UPDATE: apart from this, SkyAtmosphere has changed too and even more devastating. These scene is the entrance to a cave, that was completly dark in 5.4 (after disabling environmentafog) but now it shows a lot of light in a distance, unless I disable SkyAtmosphere

This is with both EnvironemntalFog and SkyAtmosphere enabled:


Disabled Environmental fog was enough in 5.4, but now it looks like this:

Disable SkyAtmoshere too makes is as dark as I expected to be:

Area’s above landscape seems to be free for the effects of skyAtmosphere, but the effect of EnvironmentalFog is still very promiment.: This is when it is enabled:

and this when it is disabled - not pitch black since this near an opening:

Which is like this with EnvironmentalFog enabled:

and this when disabled: