does your level have a skylight? looks to me like its trying to composite the box cubemap with the sky cubemap in a busted way. the box should take precedence in that scenario based on my understanding but its also possible I am overlooking something or missing how something was set up in your level.
if you do have a skylight try deleting or disabling it… and maybe changing it to another method (ie static if its dynamic now). if that turns out to be it, could you please make an answerhub thread? that will show it is indeed a bug with skylights.
also, does scaling the box up help? maybe its thinking you are more outside the box than you really are or something like that when in the game.