Ok, so I installed the latest beta release (build 1125) and configured proxy type 13.
Here is the log when I try to access an external ftp:
[R] Connecting to
www.xxxxx.net via Proxy -> IP=proxy.hq.k.grp PORT=21
[R] Connected to
www.xxxxx.net via Proxy
[R] 220-------------------------------
[R] 220-CORPORATE IT PROXY FTP Service
[R] 220 ------------------------------
[R] USER
sys_eboutik@www.xxxxx.net hq\userid1
[R] 331 Enter password.
[R] PASS (hidden)
[R] 332 Enter proxy password.
[R] ACCT (hidden)
[R] 230 User sys_eboutik logged in.
[R] USER sys_eboutik
[R] 503 You are already logged in!
[R] Connection failed
[R] Delaying for 120 seconds before reconnect attempt #1
Tried with a couple of known comemrcial sites (adobe, google) and get the exact same results.
If you require any other logs let me know