Not only was there backlash about the casting of Nagini, but people were furious with Johnny Depp not losing his job in the movie after his domestic violence allegations.
However, this new issue doesn't have anything to do with the casting. It's got to do with continuity issues.
Just recently, it was revealed by Warner Bros that Professor McGonnagal would be making an appearance in the movie, played by actress Fiona Glascott.
According to Pottermore, McGonagall wasn't born at the time that the movie is set, in 1927.
Fans actually note that she wasn't born in October 1935, which is eight years after she appears as a young witch (in her late 20s or early 30s) in The Crimes of Grindelwald.