I went through and got a bunch of ISRCs ready to submit, and when prompted, I entered Y (instead of y) and then it replied that nothing had been submitted.
Do you want to submit? [y/N] Y
Nothing was submitted to the server.
So I re-ran the command and when I came to the submit prompt I typed y and it worked.
Do you want to submit? [y/N] y
Successfully submitted 24 ISRCS.