Wing FTP Software

I have 5 wing ftp servers, in different domains, which send uploaded files to each other.
All works fine with this script:

require("socket.ftp")
require("socket.ltn12")

local filepath = "%PathName"
f, e = put{
host = "10.32.176.90",
user = "ftpadmin",
password = "xxxx",
port = 21,
path = "/%Name_%FileName",
source = source.file(io.open("%PathName", "rb"))
}

...except for uploading to a wing ftp server in dmz.
In dmz I get the error "550 Cannot STOR. No Permission."

I cannot get this to work, while I can upload files via a standalone ftp client to that server in dmz. Same ip, same user, same password, same port.

Authentication in dmz is via ftp user; authentication on all other servers is via Windows authentication.
The problem should be in that area, but it is so weird that I can upload via a regular client...
Wing ftp service is started by local system account in all domains and dmz.

Does anyone have an idea?

OK, it means you do not have file upload permission on that server (with that username), you had better paste the full server logs via email.