NOS FTP Server Incompatible With NeXT Client?

[bob@ke9yq.ampr.org (Bob Van Valzah, ke9yq)]

I've had two people with NeXT boxes on the Internet report that their FTP
client can't talk to my NOS server (stock 910618 ka9q w/IP access mods).
One also reported failure from a Sun.  I know it works fine with my SunOS
4.0.3 clients.  I have my tcp mss set down to 216--could that cause the
problem?  Here's the symptoms they see:

>% ftp ke9yq.imsa.edu
>Connected to ke9yq.imsa.edu.
>220 ke9yq.ampr.org FTP version 910618 ready at Tue Jan 28 15:53:30
>1992
>Name (ke9yq.imsa.edu:ecktons): kd6bik
>331 Enter PASS command
>Password:
>230 Logged in
>ftp> dir
>200 Port command okay
>150 Opening data connection for LIST /nos/public
>(lots of hang time)
>421 Service not available, remote server has closed connection

Has anybody else seen these problems?  Does anyone have a fix?  Is there
any helpful debuging info I can send other than a tcp status on the control
block after then connection has hung?

73, Bob, ke9yq  bob@ke9yq.ampr.org (yes, I have an MX record)