Errors with explicit FTP over TLS

Hello all, I'm trying to chase down the following errors we're seeing when connecting to a customer FTP over explicit FTP with TLS.

I'm not sure whether the problem is on side, or the customer side - any feedback would be useful. I've included the debug information as well

Regards

Sarty

14:47:07 Status: Connecting to ***.**.**.***:21...
14:47:07 Status: Connection established, waiting for welcome message...
14:47:07 Trace: CFtpControlSocket::OnReceive()
14:47:07 Response: 220-Microsoft FTP Service
14:47:07 Trace: CFtpControlSocket::OnReceive()
14:47:07 Response: 220 FTP.*******.COM
14:47:07 Trace: CFtpControlSocket::SendNextCommand()
14:47:07 Command: AUTH TLS
14:47:07 Trace: CFtpControlSocket::OnReceive()
14:47:07 Response: 234 AUTH command ok. Expecting TLS Negotiation.
14:47:07 Status: Initializing TLS...
14:47:07 Trace: CTlsSocket::Handshake()
14:47:07 Trace: CTlsSocket::ContinueHandshake()
14:47:07 Trace: CTlsSocket::OnSend()
14:47:07 Trace: CTlsSocket::OnRead()
14:47:07 Trace: CTlsSocket::ContinueHandshake()
14:47:28 Error: Connection timed out
14:47:28 Trace: CControlSocket::DoClose(2050)
14:47:28 Trace: CFtpControlSocket::ResetOperation(2114)
14:47:28 Trace: CControlSocket::ResetOperation(2114)
14:47:28 Error: Could not connect to server
14:47:28 Trace: CFileZillaEnginePrivate::ResetOperation(2114)