FTP connection using intergation server 6.1

First customer:
I cannot connect FTP when i start up IS 6.1 server. But when i shut down IS 6.1 server, it can FTP connect to them.
It always shows timeout connection to this customer.

Second customer:
No problem with FTP connection with them having IS 6.1 server running.

Do anyone have this situation?

Is there any solution to fix this problem?