Core FTP

We have set up a Core FTP server with SFTP enabled. When users login (with core ftp client), they are immediately disconnected. We've switched on keepalive, session timeout at 5 minutes, but nothing works. Occasionally, a user is able to stay logged on, but when logging in later, the immediate disconnect problem reoccurs again.

Can someone please help us? We've already spent days on this problem.
We're running Core FTP server on Windows 2008 Standard edition, 64-bit.

Many thanks in advance!

Jan Willem Borrius

Dear CP, many thanks for your reply. We've found the reason: another service in the cloud (we're running this in a cloud) was trying to connect every 5 seconds. We've blocked the IP address (inside Core FTP) from which this service works, and now everything is fine.
Many thanks!
Kind regards,
J.W.Borrius