I don’t have a link/haven’t found one, but apparently Austrian news network ORF Burgenland did an interview with referee Manuel Schuttengrubber where he confirmed it was the n-word
Just Googled it, and could find this whereby the ref tells the news outlet "I know nothing......nothing!" (I'm really hoping he said it in a Sgt Schultz voice)
https://burgenland.orf.at/stories/3212422/