Anonymous FTP user: "File system denied the access"

I have set up MS FTP server on Windows 8 Enterprise with anonymous access. Site has a directory in it. FTP manager shows anonymous FTP user read and write privileges to that directory. I can log in as anonymous user, but if I try to upload a file, I get the message:

200 PORT command successful.
550-Access is denied.
 Win32 error:   Access is denied.
 Error details: File system denied the access.
550 End

Any suggestions?