Does your Lethis app open up every new map with something that looks like the below image?

This is caused because your graphics is using an Nvidea setup instead of the "integrated graphics" setting. Basically, your computer is set up to play games with more complicated graphics setting than Lethis offers, so the game starts to freak out and can't display important objects as anything but black boxes.

Use the Microsoft search bar to open up an app called the Nvidea Control Panel, or a similar control panel for whatever graphics brand you have.

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Next, you're going to want to find your way to the lethis app called Lethis-Path_of_Progress.exe and switch over from the high performance Nvidea processor to the integrated graphics settings.

This exact set of steps is what worked for me personally and now lethis shows the graphics as normal, although the loading screen runs a bit longer. I wouldn't be surprised if this also helps other people with black loading screen problems, but I can't confirm that so if it happens leave a comment and I'll update the guide.