Hmpf. 
I nearly reach my limits of debugging from afar. Have you tried creating a new BP of the custom skysphere class and using this instead of the one already existing?
And have you tried doing the GetGameMode with “this” instead of “other”? :X
The thing is, the only thing that is different in my setup is, that i didn’t use a custom TriggerBox made in c++, but one just spawned in the editor and referenced inside the LevelBlueprint. So i didn’t need to get the GameMode and the currentsublevel of it. CurrentSubLevel is a custom function you made inside your GameModeClass or?
You could try recreating this and check if maybe the custom trigger is the cause.
EDIT: Yeah i just looked up the function in the documentation. I meant using “this” instead of leaving it blank