Problem connecting to FileZilla server with FTP over TLS

Hello,
I've set up a FileZilla-server (version 0.9.41 beta) on a Windows XP OS behind a D-Link router.
It runs perfectly in normal FTP-mode.
In TLS mode, however, it works only when accessed locally from within the LAN. Accessing the server from the Web ends in timing out after the connection is accepted:

(000010) 19.10.2012 12:26:24 - (not logged in) (xx.yy.zz.115)> Connected, sending welcome message...
(000010) 19.10.2012 12:26:24 - (not logged in) (xx.yy.zz.115)> AUTH TLS
(000010) 19.10.2012 12:26:24 - (not logged in) (xx.yy.zz.115)> 234 Using authentication type TLS
(000010) 19.10.2012 12:26:25 - (not logged in) (xx.yy.zz.115)> SSL connection established
(000010) 19.10.2012 12:26:25 - (not logged in) (xx.yy.zz.115)> USER testuser
(000010) 19.10.2012 12:26:25 - (not logged in) (xx.yy.zz.115)> 331 Password required for testuser
(000010) 19.10.2012 12:26:25 - (not logged in) (xx.yy.zz.115)> PASS ************
(000010) 19.10.2012 12:26:25 - testuser (xx.yy.zz.115)> 230 Logged on
(000010) 19.10.2012 12:26:25 - testuser (xx.yy.zz.115)> PBSZ 0
(000010) 19.10.2012 12:26:25 - testuser (xx.yy.zz.115)> 200 PBSZ=0
(000010) 19.10.2012 12:26:25 - testuser (xx.yy.zz.115)> PROT P
(000010) 19.10.2012 12:26:25 - testuser (xx.yy.zz.115)> 200 Protection level set to P
(000010) 19.10.2012 12:26:25 - testuser (xx.yy.zz.115)> PWD
(000010) 19.10.2012 12:26:25 - testuser (xx.yy.zz.115)> 257 "/" is current directory.
(000010) 19.10.2012 12:26:26 - testuser (xx.yy.zz.115)> TYPE I
(000010) 19.10.2012 12:26:26 - testuser (xx.yy.zz.115)> 200 Type set to I
(000010) 19.10.2012 12:26:26 - testuser (xx.yy.zz.115)> PASV
(000010) 19.10.2012 12:26:26 - testuser (xx.yy.zz.115)> 227 Entering Passive Mode (xx,yy,zz,115,19,146)
(000010) 19.10.2012 12:26:26 - testuser (xx.yy.zz.115)> MLSD
(000010) 19.10.2012 12:26:26 - testuser (xx.yy.zz.115)> 150 Connection accepted
(000010) 19.10.2012 12:27:27 - testuser (xx.yy.zz.115)> disconnected.

Turning off the windows firewall doesn't help and operating the server in a DMZ doesn't work either.
I have configured FileZilla to use ports 5000-5100 in passive mode and to retrieve external IP addresses. The Router is set to forward Ports 21, 990 and 5000- 5100 to the server.

I would be very grateful for any suggestions anyone can provide.
Thanks