This bug is very old. When WinSCP connects to a mainframe via FTP, it assumes a unix like machine. This does not work and fails. Here is the log:

Using FTP protocol.
Doing startup conversation with host.
PWD
257 "'X12345.'" is working directory.
Getting current directory name.
Retrieving directory listing...
CWD /'X12345.'/'X12345.'/
550 Command CWD fails: access to resource is denied by server.
Could not retrieve directory listing
Retrieving directory listing...
CWD /'X12345.'/'X12345.'/
550 Command CWD fails: access to resource is denied by server.
Could not retrieve directory listing
(ECommand) Error listing directory ''X12345.''.
Could not retrieve directory listing
Command CWD fails: access to resource is denied by server.
Startup conversation with host finished.

Any chance of getting a test account on your server?

I am trying to connect to a mainframe using WINSCP 4.3.8.1817
I login correctly and then I get the error message:

Errror changing directory to "XAA0FT5.".
CWD cmd failed : EDC5129I NO SUCH FILE OR DIRECTORY. (errno2=0x05190050)

I found a few old posts referring this problem, but no solution.

Any ideas?