A wild guess is that maybe something in the config.txt file has a path statement mixed up. But horns don't usually have subfolders. Does the whole error message give the path in question?
I have seen that error before. Definitely check the path in the config.txt file. I had a bunch of horns from that website that are faulty due to the author using an explicit path located on their own computer similar to this here that I made up to show you as an example.
C:\Users\steven\Documents\Hornz\K3LA\k3la.wav
or ././/./Dell/users/steven/documents/hornz/K3LA/k3la.wav
What you need to do is remove the path part and leave only the k3la.wav
