A common ftp package for linux is proftp.
However nUpdate Administration does throw a error when connecting to such a server with default configuration.
I traced the error down and the problem seems like nUpdate Administration client did not reuse SSL session and therefore get's a rejecting data connection.
The workaround then, is to add the TLSOption to your configuration, i.e.:
# Relax the requirement that the SSL session be reused for data transfers
TLSOptions NoSessionReuseRequired
This is more a information for you - perhaps take a closer look how you handle ssl sessions.