This one is a fun one... short answer being it is an IE problem.
This is outlined in: http://technet.microsoft.com/en-us/library/bb794745.aspx, see the section on "How to access an FTP site that is not anonymous using Internet Explorer". The article is written for ISA servers, but applies to any type of proxy.
In the case of the Web Gateway 7.x, it will send a 401 using the following response in order to get the credentials from the user, here is what it looks like: See IE.png.
As you can see there is never a response from IE with the basic credentials.
If you were to test with Firefox, it would work. Here is a excerpt: See FF.png.
This is because Firefox will receive the 401 response for ftp:// URLs from the proxy properly, whereas IE will not.
we have one customer that has the same error, when he access one FTP site that request credentials occours the error, like the image attached.
When we disable the policy AntiMalware he works fine.
You can help us?
Gustavo Lima Miranda
Diretor de Operações
Tel.: +55 (61) 3363-8636
FTP-ELN.jpg 214.8 K