'Invalid segment retransmission. Packet dropped.'

[Tim_Bernat]

Hi All, 

we have a client not able to connect to an FTP server. The connection goes through the internal firewall and then gets dropped by our external CP (80.10). The sync packet is okay, but then it is actually dropped by the same rule that should be allowing it with the 'Invalid segment retransmission. Packet dropped.' comment. Please see the below screen.

We initially thought it was down to the application (FileZilla), but it seems it's the same, for example, from win command line. 

Thank you for any comments.