Can't connect to FTP server

I am trying to get the NAFTPAPP program to work on my board. The only change i have done is that the program uses my BSP (which I have tested with my own program which work fine) and the speed has been set to 10Mb/s instead of 100Mb/s (I do this by calling mii_set_speed (0, 1))). I can ping the board and is responds but when either the NETARM FTP program or an ordinary FTP client tries to connect to the ftp server the FTP server responds but with the ACK and the RST (reset the connection) flag set in the TCP header. All the registrations and the start of the ftp server reports that all is ok. None of the registrated routines get called so the ftp server discards the packet at a low level. I have been able to connect to the server 4-5 times but most of the time I can’t. Why can’t I connect to the FTP server??

Don’t answer that I can’t connect since the RST bit is set in the header. I want to know why the FTP server sets the RST bit, what causes it and what I shall do to solve it.