At times I've had to SET PASV or set passive mode on some connection/router issues. So try passive mode and then ask CoreFTP for more ideas.
We are experiencing an issue where one external site cannot connect to our SFTP server. It works for all other customers and is only affecting this one site when they're connecting from their internal network. They've tested connecting to an SFTP server in a different location (one of ours) which is running the same FTP server and that one works. They have tested from the same machine on an external network and it works fine. A wireshark capture on the Windows FTP server is showing that the traffic arrives but then sends a FIN packet back to the client. Looking at the Application logs, we're not seeing any traffic coming in. On a successful SFTP connection, we normally would see the traffic on the Application logs.
They're currently using CoreFTP and have tested with other FTP clients but no difference is seen. We've tried issuing new credentials but still no change.
Any ideas or suggestions on what could be the issue?