I had the same issue. Seems an ongoing issue with Microsoft, even with SOME backwards compatible games playing on newer consoles. What worked for me was clearing the cash and deleting the latest system update. I couldn't get online after that, so deleted one of the profiles on the 360 and then...
My son actually wanted to play Rock Band/ Guitar Hero, on the 360, after they introduced Fortnite Festival. Now he wants to get the Rock Band 4 guitar. Those things shot up in price quickly now that they work in Fortnite. Used ones are going for at least $200. I could have gotten those at $25...