There may be more than one reason for this. Known reasons:
This is normal if you’re running the game with OpenGL. You need to download Black Sky Fix Add-on to fix this issue. For add-on users: You may be using outdated versions of the add-ons. If you download an update of RGM, you need to check if there is a new version available for the plugins you use. View add-ons for RGM Other mods you use may be causing this problem. Disable all your other mods and try this mod again.