Although replying to an old post just thought it might help someone.
When you create your ftp url make sure you are not including the default directory for that login.
for example this was the path which I was specifying and i was getting the exception 553 FileName not allowed exception
ftp://myftpip/gold/central_p2/inbound/article_list/jobs/abc.txt
The login which i used had the default directory gold/central_p2.so the supplied url became invalid as it was trying to locate the whole path in the default directory.I amended my url string accordingly and was able to get rid of the exception.
my amended url looked like
ftp://myftpip/inbound/article_list/jobs/abc.txt
Thanks,
Sab