Light maps not baking - ALL black 4.25

Hi all, my first post here, I’m relatively new to Unreal.

I have an issue where all the shadows in a level (even the most basic one) are rendering solid black, there is no indirect lighting what so ever.

I’ve researched a lot and found a few people saying add in a skylight. I have done that.

I have found that the light maps are baking completely black - could this be the issue?—and is there a solution?

Here is a level I have had no problem with, the shadows render as they should and the light maps show all the coloured areas you would expect.

And here is a level where you can see the issue with the shadows. See the light maps are completely black.

Can anyone help.

FYI. this is happening to a college too, we have the same iMac Pro with the following spec

The OK level is 4.25 and the problem is 4.25. If our 4.24 levels are opened in 4.25 this problem occurs. Also when we work further on a 4.24 level, the map baking issue occurs.