FTP PASV Error if "masqueradeAddress" is set

Hello,

first of all i think i've choosen the correct section

I installed a fresh i-mscp (1.2.17) on my server that is behind a firewall.
It has another internal IP than the external.
I configured the external IP as NAT IP.

My FTP Config has this line
MasqueradeAddress <Public / External IP>

If this line is set, i get the following error while connecting:
The server returned invalid response for PASV command.

If this line is commented and I restart the server the FTP works fine.
The Public ip is correct and im directly connecting via ip not hostname or sth.

Any suggesstion how to fix this problem?
(Sure i could comment the line and restart, but if the files where built new, the error exists again)

im using the ProFTPD Server

FTP Client Log
Using FTP protocol.
Doing startup conversation with host.
PWD
257 "/" is the current directory
Getting current directory name.
Rufe Verzeichnisinhalt ab...
TYPE A
200 Type set to A
PASV
500 The server returned invalid response for PASV command.
Konnte Verzeichnisinhalt nicht abrufen
(ECommand) Fehler beim Anzeigen des Verzeichnisses "/".
Konnte Verzeichnisinhalt nicht abrufen
The server returned invalid response for PASV command.
Startup conversation with host finished.

(Sorry that its German)

connecting with winscp
host: public IP adress
tracert returns one hop (so the firewall seems to route the ping directly to the server, because the time is less than 1ms)
im connecting from the LAN Network

The server is not at home but in the lan network (same subnet) where im in.