IIS 6.0 FTP Issue

I am having some users able to connect to my FTP server and upload files, but I have a problem user and their client is giving them this error:
An existing connection was forcibly closed by the remote host.

I have a stanard IIS 6.0 install on Windows 2003 SP1
Users get locked to their home directory by the name of the user account matching the home directory name
I had some deny IPs in IIS, I took those out
I have restarted the service
I can log into the FTP as this user from another internet connection
Customer’s client is CoreFTP LE

Any ideas as to what is disconnecting this one user?