I have no idea how plesk works as haven't used it for long. However, can you check the folder permissions manually through RDP on server? Check that FTP user has Read/write/modify permissions on the folder.
██ Softsys Hosting ██ - 8 Years In Business - 24/7 In-House Support - Enterprise DDoS Protection Available!
I'm tempted to say its an issue with Windows firewall and the passive port range not being set. As a quick test, disable Windows firewall and see if you're able to get the directory listing. You can either do this manually or I'm pretty sure Plesk 9 has options to control the firewall when you're logged in as the primary system admin.
Applied Innovations (www.appliedi.net)
Microsoft Gold Certified Hosting Partner, ASP.net Featured Host.
Specializing in Windows Hosting since 1999.