OK, there is a problem several people encountered, of skybox turning black when you reload a save. I don’t know how to prevent it, but there is a way to restore the skybox.
Found it here: https://developer.valvesoftware.com/wiki/Sky_List
What you do is enter sv_skyname <sky_name> in the console. You find the list of skyboxes in the skybox folder. Then you save and reload.