the ftp.Abort() method invokes ABOR command (RFC 959):
... The control connection is not to be closed by the server, but the data connection must be closed."
Unfortunately, some FTP servers cannot handle this command appropriately and closes the control connection as well.
If you want to diagnose what is going on with your server, please create communication log. If you need help with diagnosing post the log here or send it to email@example.com.
If it shows that the client closes the control connection, it is a bug in FTP client.
If it shows that the server closes the connection, it is bug in FTP server and the server has to be fixed (it cannot be workarounded on client side).