B4J Question FTP server port 21

Sub Process_Globals
    Private server As FTPServer
End Sub

Sub AppStart (Args() As String)
    server.Initialize(Me, "FTPServer")
    server.SetPorts(99, 51045, 51057)
    server.AddUser("dddd", "dddd")
    server.Start
    StartMessageLoop
End Sub

Sub FTPServer_StateChanged
    Log($"Number of clients: ${server.NumberOfClients}"$)
End Sub

server.SetPorts(21, 51045, 51057)

Number of clients: 0
terminated
Number of clients: 1
User logged in: dddd
CurrentPath: F:/PROG_19/ _____B4J/FTP serwer
java.net.SocketException: Software caused connection abort: socket write error
    at java.net.SocketOutputStream.socketWrite0(Native Method)
    at java.net.SocketOutputStream.socketWrite(SocketOutputStream.java:111)
    at java.net.SocketOutputStream.write(SocketOutputStream.java:143)
    at anywheresoftware.b4a.randomaccessfile.AsyncStreams$AOUT.run(AsyncStreams.java:355)
    at java.lang.Thread.run(Thread.java:748)
java.net.SocketException: Software caused connection abort: recv failed
    at java.net.SocketInputStream.socketRead0(Native Method)
    at java.net.SocketInputStream.socketRead(SocketInputStream.java:116)
    at java.net.SocketInputStream.read(SocketInputStream.java:171)
    at java.net.SocketInputStream.read(SocketInputStream.java:141)
    at java.net.SocketInputStream.read(SocketInputStream.java:127)
    at anywheresoftware.b4a.randomaccessfile.AsyncStreams$AIN.run(AsyncStreams.java:216)
    at java.lang.Thread.run(Thread.java:748)
error: (SocketException) java.net.SocketException: Software caused connection abort: recv failed
error: (SocketException) java.net.SocketException: Software caused connection abort: recv failed
terminated
Number of clients: 0
terminated