Thank you in advance for your help.
Here's the debug output:
Trace: FtpControlSocket.cpp(4657): m_pSslLayer changed state from 0 to 1 caller=0x003ac6d4
Status: Connecting to x.y ...
Trace: FtpControlSocket.cpp(4657): m_pSslLayer changed state from 1 to 2 caller=0x003ac6d4
Trace: FtpControlSocket.cpp(4657): m_pSslLayer changed state from 2 to 4 caller=0x003ac6d4
Trace: FtpControlSocket.cpp(772): OnConnect(0) OpMode=1 OpState=-8 caller=0x003ac6d4
Status: Connected with x.y, negotiating SSL connection...
Trace: FtpControlSocket.cpp(640): OnReceive(0) OpMode=1 OpState=-8 caller=0x003ac6d4
Response: 220 x.y FTP server ready
Command: AUTH TLS
Trace: FtpControlSocket.cpp(640): OnReceive(0) OpMode=1 OpState=-9 caller=0x003ac6d4
Response: 234 AUTH TLS successful
Trace: SSL_connect:before/connect initialization
Trace: SSL_connect:SSLv2/v3 write client hello A
Trace: SSL_connect:SSLv3 read server hello A
Trace: SSL_connect:SSLv3 read server certificate A
Trace: SSL_connect:SSLv3 read server key exchange A
Trace: SSL_connect:SSLv3 read server done A
Trace: SSL_connect:SSLv3 write client key exchange A
Trace: SSL_connect:SSLv3 write change cipher spec A
Trace: SSL_connect:SSLv3 write finished A
Trace: SSL_connect:SSLv3 flush data
Trace: SSL_connect:SSLv3 read finished A
Trace: Using TLSv1, cipher TLSv1/SSLv3: DHE-RSA-AES256-SHA, 1024 bit RSA
Status: SSL connection established. Waiting for welcome message...
Command: PBSZ 0
Trace: FtpControlSocket.cpp(640): OnReceive(0) OpMode=1 OpState=-11 caller=0x003ac6d4
Response: 200 PBSZ 0 successful
Command: PROT P
Trace: FtpControlSocket.cpp(640): OnReceive(0) OpMode=1 OpState=-12 caller=0x003ac6d4
Response: 530 Please login with USER and PASS
Command: USER username
Trace: FtpControlSocket.cpp(640): OnReceive(0) OpMode=1 OpState=0 caller=0x003ac6d4
Response: 200 Protection set to Private
Command: FEAT
Trace: FtpControlSocket.cpp(640): OnReceive(0) OpMode=1 OpState=-13 caller=0x003ac6d4
Response: 331 Password required for username.
Command: SYST
Trace: FtpControlSocket.cpp(640): OnReceive(0) OpMode=1 OpState=-14 caller=0x003ac6d4
Response: 211-Features:
Trace: FtpControlSocket.cpp(640): OnReceive(0) OpMode=1 OpState=-14 caller=0x003ac6d4
Response: MDTM
Trace: FtpControlSocket.cpp(640): OnReceive(0) OpMode=1 OpState=-14 caller=0x003ac6d4
Response: REST STREAM
Trace: FtpControlSocket.cpp(640): OnReceive(0) OpMode=1 OpState=-14 caller=0x003ac6d4
Response: SIZE
Trace: FtpControlSocket.cpp(640): OnReceive(0) OpMode=1 OpState=-14 caller=0x003ac6d4
Response: AUTH TLS
Trace: FtpControlSocket.cpp(640): OnReceive(0) OpMode=1 OpState=-14 caller=0x003ac6d4
Response: PBSZ
Trace: FtpControlSocket.cpp(640): OnReceive(0) OpMode=1 OpState=-14 caller=0x003ac6d4
Response: PROT
Trace: FtpControlSocket.cpp(640): OnReceive(0) OpMode=1 OpState=-14 caller=0x003ac6d4
Response: 211 End
Status: Connected
Trace: FtpControlSocket.cpp(3502): ResetOperation(1) OpMode=1 OpState=-14 caller=0x003ac6d4
Trace: FtpControlSocket.cpp(640): OnReceive(0) OpMode=0 OpState=-1 caller=0x003ac6d4
Response: 215 UNIX Type: L8
Trace: FtpControlSocket.cpp(917): List(FALSE,0,"","",1) OpMode=0 OpState=-1 caller=0x003ac6d4
Status: Retrieving directory listing...
Command: PWD
Trace: FtpControlSocket.cpp(640): OnReceive(0) OpMode=4 OpState=0 caller=0x003ac6d4
Response: 530 Please login with USER and PASS
Trace: FtpControlSocket.cpp(917): List(FALSE,0,"","",0) OpMode=4 OpState=0 caller=0x003ac6d4
Trace: FtpControlSocket.cpp(3502): ResetOperation(4) OpMode=4 OpState=7 caller=0x003ac6d4
Error: Could not retrieve directory listing
Trace: FtpControlSocket.cpp(834): DoClose(0) OpMode=1024 OpState=-1 caller=0x003ac6d4
Trace: FtpControlSocket.cpp(3502): ResetOperation(4100) OpMode=1024 OpState=-1 caller=0x003ac6d4
Trace: SSL3 alert write
Trace: FtpControlSocket.cpp(4657): m_pSslLayer changed state from 4 to 0 caller=0x003ac6d4
Status: Disconnected from server