[Zope] Problems with FTP and Windows 98 clients behind a "firewall" ...

Hi! Currently we have problems using FTP from Windows 98 on Zope 2.3.2: Whenever I try to open a Zope server's FTP port from the in-built MS IE 5.0 FTP support or from teh command-line FTP, I can't get any files listed. "ls" will just hang. This was not a problem with older Zope versions as far as I can recall (But it could also be caused by the firewall). FTP also seems to work fine from Linux (command-line and Konqueror). A further investigation showed that the problem seems to be caused by the "firewall" configuration we are using and the use of "passive mode FTP". Anybody experienced this, too? Any solutions? Joachim