Hallo Christoph,
vielleicht ist dies ein Ansatz ?

FTP Connection Ending Abnormally
Incident Summary

Problem Summary:
FTP client connection from the IBM iSeries™ family of servers or the IBM System i5™ is periodically failing with a reply code 425. PORT mode is being used.

Resolution:
Information APAR II13451 indicates that this is occurring because the IBM iSeries™ family of servers or the IBM System i5™ is reusing a data port too quickly and, therefore, not allowing the connection to be cleaned up on the server side. Following is the explanation provided in the INFO APAR and ways to work around the problem.

iSeries FTP client and server were changed in V5R1 to assign ephemeral port numbers randomly. This change was made to prevent possible security exposures which can take place if a "hacker" can predict the next port number which TCP will use, as this information can be used to carry out a "spoofing" attack. Assigning port numbers randomly is suggested in RFC 2577 - FTP Security Considerations. However, the sequence of port numbers is not always "random" across FTP client and server jobs which causes intermittent failures in establishing data connections on high-use systems. Because of these security enhancements, users running FTP client connections in PORT mode may see intermittent problems with data connections. The remote system may send a " 425 " error.

The following can be used to avoid the problem:
o Use Passive Mode connection ("SENDPASV 1")
o Monitor for the error message and retry the operation.
o Change the TCP closed connection wait (TIMW) timeout value on the remote system (for example, the system running the FTP server) to a very small value (for example, 1 sec) may also resolve the problem.