So I had FTP setup on a Server '03 box with an FTP site configured to use user isolation mode with active directory. Everything was working great then all of a sudden users cant login and this error appears in the System events in the event log:
This error, while true, doesn't really pinpoint the problem... But I found this wonderful article that suggested that the account used by the FTP site to access AD was changed. "Oh yeah!", I said, "I did change that, D'OH!". Easy enough to fix; just open a command prompt and use the adsutil.vbs to update the AD account in the IIS metabase.
c:\Inetpub\Adminscripts\adsutil.vbs set msftpsvc/{FTP_SITE_ID}/ADConnectionsUserName MyDomain\MyFTPADUsername
c:\Inetpub\Adminscripts\adsutil.vbs set msftpsvc/{FTP_SITE_ID}/ADConnectionsPassword S0m3P@$$W0rd
Remember to include the domain name with the username.