FileZilla not working with box.com ftp | debug help

[johnny_canuck]

So, been using FZ for years -- current incarnation (3.48.1) under Windows 10 (Home and Pro). One of the servers I point the FZ client at is box.com. Normally works fine, but as of today, no such luck. Normally, you simply use the following config:

Server: ftp.box.com
Username: Your Box account email address (SSO log in not supported)
Password: Your Box account password
Port: <leave blank>
For implicit encrypted connection (FTPS): 990
For standard, unencrypted connection (FTP) or explicit encrypted connection (FTPES): 21. Transfers may be slower when using encryption due to additional processing.
TLSv1.2 is required. TLSv1.0 and TLS 1.1 are no longer supported.

I've been using implict over 990 for years - no problem. But not, absolutely refuses to connect. Every other client I've tried (gFTP, WinSCP, CyberDuck, etc) all work *perfectly*. Just not FileZilla...

Before someone says 'its a problem with box.com' (which doesn't easily explain with FZ is the *only* client I've tried that fails), I'm hoping someone can help me debug things, so I can communicate same to box.com support. I've set debug level to verbose, and dumped session details into a log (edited version of same below), but...is there anything else I should consider fooling with, or recroding in some fashion?

Thanks in advance...

2020-07-12 21:21:46 8084 1 Trace: CControlSocket::SendNextCommand()
2020-07-12 21:21:46 8084 1 Trace: CFtpLogonOpData::Send() in state 0
2020-07-12 21:21:46 8084 1 Status: Resolving address of ftp.box.com
2020-07-12 21:21:46 8084 1 Status: Connecting to 107.152.27.220:21...
2020-07-12 21:21:46 8084 1 Status: Connection established, waiting for welcome message...
2020-07-12 21:21:46 8084 1 Trace: CFtpControlSocket::OnReceive()
2020-07-12 21:21:46 8084 1 Response: 220 Service ready for new user.
2020-07-12 21:21:46 8084 1 Trace: CFtpLogonOpData::ParseResponse() in state 1
2020-07-12 21:21:46 8084 1 Trace: CControlSocket::SendNextCommand()
2020-07-12 21:21:46 8084 1 Trace: CFtpLogonOpData::Send() in state 2
2020-07-12 21:21:46 8084 1 Command: AUTH TLS
2020-07-12 21:21:46 8084 1 Trace: CFtpControlSocket::OnReceive()
2020-07-12 21:21:46 8084 1 Response: 234 Command AUTH okay; starting TLS connection.
2020-07-12 21:21:46 8084 1 Trace: CFtpLogonOpData::ParseResponse() in state 2
2020-07-12 21:21:46 8084 1 Status: Initializing TLS...
2020-07-12 21:21:46 8084 1 Trace: tls_layer_impl::client_handshake()
2020-07-12 21:21:46 8084 1 Trace: tls_layer_impl::continue_handshake()
2020-07-12 21:21:46 8084 1 Trace: tls_layer_impl::continue_handshake()
2020-07-12 21:21:46 8084 1 Trace: tls_layer_impl::continue_handshake()
2020-07-12 21:21:46 8084 1 Trace: tls_layer_impl::continue_handshake()
2020-07-12 21:21:46 8084 1 Trace: tls_layer_impl::continue_handshake()
2020-07-12 21:21:46 8084 1 Trace: tls_layer_impl::continue_handshake()
2020-07-12 21:21:46 8084 1 Trace: TLS Handshake successful
2020-07-12 21:21:46 8084 1 Trace: Protocol: TLS1.2, Key exchange: ECDHE-SECP384R1-RSA-SHA384, Cipher: AES-256-GCM, MAC: AEAD
2020-07-12 21:21:46 8084 1 Trace: tls_layer_impl::verify_certificate()
2020-07-12 21:21:46 8084 1 Status: Verifying certificate...
2020-07-12 21:21:46 8084 1 Trace: CFtpControlSocket::SetAsyncRequestReply
2020-07-12 21:21:46 8084 1 Status: TLS connection established.
2020-07-12 21:21:46 8084 1 Trace: CControlSocket::SendNextCommand()
2020-07-12 21:21:46 8084 1 Trace: CFtpLogonOpData::Send() in state 6
2020-07-12 21:21:46 8084 1 Command: USER user.name@whatever.com
2020-07-12 21:21:46 8084 1 Trace: CFtpControlSocket::OnReceive()
2020-07-12 21:21:46 8084 1 Error: Connection closed by server
2020-07-12 21:21:46 8084 1 Trace: CFtpControlSocket::ResetOperation(66)
2020-07-12 21:21:46 8084 1 Trace: CControlSocket::ResetOperation(66)
2020-07-12 21:21:46 8084 1 Trace: CFtpLogonOpData::Reset(66) in state 6
2020-07-12 21:21:46 8084 1 Error: Could not connect to server

[botg]

What is the displayed certificate fingerprint in the certificate verification dialog? You should be able to access it through the lock icon in the status bar that briefly shows.

[johnny_canuck]

What is the displayed certificate fingerprint in the certificate verification dialog? You should be able to access it through the lock icon in the status bar that briefly shows.

OK -- thanks. I'll try that and report back when I get a chance.

[johnny_canuck]

Well, in fact, there is no 'lock' icon, because it never gets that far. So, no certificate information to provide.

[johnny_canuck]

So, I blew away the certs .xml file, and tried connecting to box.com again. Whats interesting is that FZ pops up a window saying ''Unknwon certificate'. I went ahead an 'accepted' it, but FZ still refuses to connect to box.com.

Now, the added wrinkle is, when I weent into my office this morning (University network), FZ connected to box just fine. But, at home, using Spectrum as my ISP - nada. Configurations are identical.

[botg]

Okay. But what was the displayed fingerprint?