It is rather complicated.They are saying that the show was criticized because there was no balance of races in in. So until they changed the balance (of people) in the show there was a lot of criticism. The way it is written is very unclear in my opinion. I don't see any difference in meaning between the two versions of the sentence. Is it written exactly like that in the article? "Until" here means "up to" . "Before" means "in the past" so the meaning is approximately the same. For example "I lived in London until I moved to Manchester". "I lived in London before I moved to Manchester". THe meaning is identical. WE can say "the racial imbalance problem" or "the problem of racial imbalance" or "the problem with racial imbalance". It doesn't really matter. It is just a question of style.