You're probably still working with the 1.0.1 version of ftplib.py. In
1.0.2, all calls to listen(0) have been changed to listen(1). This
was necessary for Solaris 2; using listen(0) there gave exactly the
same kind of error you are reporting here.
--Guido van Rossum, CWI, Amsterdam <Guido.van.Rossum@cwi.nl>
PS thanks for the contribution!