Hey try using the built in USB loader within the editor. Horizon has been known to cause issues with the save causing the modded dark souls 2 save to become
Corrupt and no longer able to save. Last time I use horizon for a dark souls save it change the size if the file from 14.06 mb to 8.96 mb which cause a problem when you try to replace it in horizon as well and brought the problems I mentioned above. If you need help I will try and help you.