I've noticed that if I run the server as a service, clients get a pop-up on an attempted access:
FTP Folder Error
Windows cannot access this folder. Make sure you typed the file name correctly and that you have permission to access the folder.
Details:
A connection with the server could not be established.
If I stop the service and quit, then run the server from the Start menu, it works fine. Clients get the logon form and with correct id and password, access is granted.