Looks like some race condition-style bug in the server.

I have sent you an email with a debug version of WinSCP to the address you have used to register on this forum.

Please use the debug version for both the GUI and assembly test.

Here is the Log from the GUI WinSCP Version 5.9.2 (Build 6958) (OS 10.0.14393 - Windows 10 Pro)
I am using the same .NET Assembly WinSCP Version 5.9.2 (Build 6958) (OS 10.0.14393 - Windows 10 Pro)
Looks like I missed the Line with:>> 500 Command "" not recognized << after the SYST in the assembly log..

Connected with 89.XXX.XXX.166:2101. Waiting for welcome message...
220 FTP server (Version wu-1.0.1) ready.
USER blubb
331 Password required for btbhdl.
PASS ************
230-HostID(SW:16.04.3 16.2, HW:C-10.10, SN:927)
230 User blubb logged in.
SYST
215 UNIX Type: L8
FEAT
500 Command "FEAT" not recognized.
Session upkeep
Connected
Got reply 1 to the command 1
--------------------------------------------------------------------------
Using FTP protocol.
Doing startup conversation with host.
PWD
257 "/WPRAM/PROT/BTB" is current directory.
Got reply 1 to the command 16
Getting current directory name.
Retrieving directory listing...




Weird thing is, I have this error on 13 FTP out of about 60 FTP Servern.
They are all the same embedded system, same hardware, same software, same FTP server.
and only when I use the assembly. Using the Programm directly or with a script all goes well.

If there is anything else you might need to identify the cause please let me know.

Regards
Jorg



EDIT:
---------------------------
Turns out, the Password list I received for these 13 Servers had errors.
After the actual password was a carriage return and a line feed
which was probably interpreted as the "" command.
Those 'bad' passwords where copied into the database where the Programm picks up the credentials.
using the GUI I entered the password by hand so the error was not carried over hence did not exist.

I am really sorry to have bothered you with this!!
Regards
Jorg

The problem is the stray "500 Command "" not recognized." response after the SYST command. It de-synchronizes pairing commands with responses.

I cannot imaging, how the GUI can recover from this. Show us a log file.

When connecting to an FTP server via .net assembly the connection gets closed after the FEAT command gets issued:

Connected with XXX.XXX.XXX.XXX:2101. Waiting for welcome message...
220 FTP server (Version wu-1.0.1) ready.
USER blubb
331 Password required for blubb.
PASS **************
230-HostID(SW:16.04.3 16.2, HW:C-10.10, SN:927)
230 User blubb logged in.
SYST
500 Command "" not recognized.
FEAT
215 UNIX Type: L8
Connected
--------------------------------------------------------------------------
Using FTP protocol.
Doing startup conversation with host.
PWD
500 Command "FEAT" not recognized.
Getting current directory name.
PWD
257 "/WPRAM/PROT/BTB" is current directory.
Attempt to close connection due to fatal exception:
(ExtException) **Command "FEAT" not recognized.**
Disconnected from server

Using the GUI I get the same log entries up to '... is current directory'
but then no exception and the session stays open
Any Idea on how to avoid this in .net


Thanks for any help
Jorg