Note: You can access the FTP server through IE without any issue.
When configured to use Explicit Proxy, FileZilla uses the HTTP CONNECT method on port 21.
The request is similar to “CONNECT FTP_server:21” and sometimes the proxy blocks the CONNECT method on ports other than 443 (SSL).
To allow the HTTP CONNECT method, add the below CPL code in your local policy file.
<proxy> http.method=CONNECT url.port=21 detect_protocol(none) allow