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

when I try to log on to an FTP server at ITU.int and after I enter my username and password, the following window pops up.

Invalid access to memory.

Please help us improving WinSCP by reporting the error on WinSCP support forum.
WinSCP 5.9.5

Error message:
Error listing directory '/'.

Invalid access to memory.

Stack trace:
(00855F28) C3463_0
(0086700E) C5073_3
(00869076) ____ExceptionHandler
(00865E30) __ExceptionHandler
(000851CF) ntdll.dll
(00073161) ntdll.dll.KiUserExceptionDispatcher
(00608F3C) _ne_sspi_create_context
(005FF6FF) C2373_0
(005FF787) C2373_0
(00600A2B) C2373_0
(00600DB5) C2373_0
(00604F18) _ne_end_request
(0060506E) _ne_request_dispatch
(0060BE03) _ne_propfind_get_request
(0060BEE7) _ne_propfind_allprop
(00790247) TWebDAVFileSystem::ReadDirectoryInternal
(0079043E) TWebDAVFileSystem::ReadDirectory
(00778643) TTerminal::CustomReadDirectory
(00777BD1) TTerminal::ReadDirectory
(00777050) TTerminal::ReloadDirectory
(006551D4) TUnixDirView::ReloadDirectory
(00038277) TNonVisualDataModule::ExplorerActionsExecute
(003B5388) System::Actions::TContainedActionList::ExecuteAction
(0019C501) Vcl::Actnlist::TCustomAction::Execute
(0037326C) System::Classes::_18201
(00031B59) USER32.dll
(000240EE) USER32.dll
(00023A7D) USER32.dll
(0002384B) USER32.dll.DispatchMessageW

(00867679) C5073_3
(00868C53) _ReThrowException
(00778672) TTerminal::CustomReadDirectory
(00777BD1) TTerminal::ReadDirectory
(00777050) TTerminal::ReloadDirectory
(006551D4) TUnixDirView::ReloadDirectory
(00038277) TNonVisualDataModule::ExplorerActionsExecute
(003B5388) System::Actions::TContainedActionList::ExecuteAction
(0019C501) Vcl::Actnlist::TCustomAction::Execute
(0037326C) S

I know how to reproduce the problem or the problem happens frequently enough. I wish to be contacted by the WinSCP team to help resolving the problem.