passive FTP via VIP64 is not working

Hello 

I have a problem with the passive FTP transfer. It gets connected but the data transfer does not work. Active FTP works fine. Behind the router I have several FTP server which are addressed with the same external IP but different ports. This works fine so far for sftp, http,... But not for FTP in passive mode. The setup is the following:

A NAT64 VIP is translating from IPv6 into IPv4 and also does a port forwarding. External port = 6221 into internal 21. The standard session-help is in place and allows the active FTP transfer. If I change that, active FTP stops working. 

    edit 9
        set name ftp
        set protocol 6
        set port 21
    next

The NAT64 VIP is also added in a NAT64 policy where "Use Outgoing Interface Address" is activated. For the test setup all internal to WAN traffic and services are allowed. 

When I use active or passive with IPv4 everything works fine. 

Is there something I am missing?

Marcus