FtpWebRequest Supports Downloads Through an HTTP Proxy, but Not Uploads
Read FtpWebRequest's documented HTTP-proxy operation limits before changing passwords or assuming a successful listing proves upload support.
A directory listing and download can work through an HTTP proxy while FtpWebRequest rejects UploadFile. That is a documented capability limit, not proof of a bad password.
For an HTTP proxy, its supported operations are DownloadFile, ListDirectory and ListDirectoryDetails. Enabling passive mode does not add upload support to that API.
A capable HTTP CONNECT path can carry another FTP client's TCP connections, subject to proxy policy. An HTTP-to-FTP gateway can offer its own operations. Neither makes every FTP API implement every arrangement.
Setting Proxy to null can make a permitted direct upload work, but that bypasses the intermediary. It has not repaired a proxied upload.
Choose a supported FTP client and route for the required operation. FtpWebRequest is obsolete for new development; HttpClient is not, by itself, an FTP replacement. Only after establishing that the operation is supported should you investigate its proxy credentials, FTP credentials and data connections.