My daughter is having the same problem today. All was working well and then now it isn't. We purchased three Aphmau mods and downloaded them, I don't know if that has anythiing to do with it? My daughter can't create any new worlds or even enter save worlds without getting the message "There was a problem loading pack one of your packs".
Thank you for your help!
ETA: I figured it out. She had a texture pack and I had to activate it and then deactivate it.
