<Try to describe precise steps that lead to the problem (where do you click, what keys do you press, what do you see, etc.)>
Well, just tried to zip up a small text file to send for some debugging of a firewall issue...
<If relevant, consider attaching a session log file or a screenshot)>

WinSCP 5.9.2

Error message:
Invalid access to memory.

Stack trace:
(003E625C) System::UStrEqual
(00863846) C5073_3
(008658AE) ____ExceptionHandler
(00862668) __ExceptionHandler
(0003EE3E) ntdll.dll.ZwWow64CallFunction64
(0003F5BA) ntdll.dll.KiUserExceptionDispatcher
(006CE201) TRemoteFile::GetIsParentDirectory
(006D1A14) TRemoteFile::GetFullFileName
(007790C8) TTerminal::CalculateFileSize
(00776AF9) TTerminal::ProcessFiles
(00779526) TTerminal::CalculateFilesSize
(00785271) TTerminal::CopyToLocal
(0001156D) TCustomScpExplorerForm::TemporarilyDownloadFiles
(0000994E) TCustomScpExplorerForm::CustomCommand
(0000E6F1) TCustomScpExplorerForm::ExecuteFileOperation
(0000E91C) TCustomScpExplorerForm::ExecuteFileOperation
(0000E9A3) TCustomScpExplorerForm::ExecuteFileOperationCommand
(0003DAE4) TNonVisualDataModule::CustomCommandClick
(00372BEC) System::Classes::_18201
(00007E6F) USER32.DLL.CallNextHookEx
(000080CC) USER32.DLL
(0000966A) USER32.DLL
(000096DB) USER32.DLL.DispatchMessageW

(003E625C) System::UStrEqual
(00863846) C5073_3
(008658AE) ____ExceptionHandler
(00862668) __ExceptionHandler
(0003EE3E) ntdll.dll.ZwWow64CallFunction64
(0003F5BA) ntdll.dll.KiUserExceptionDispatcher
(006CE201) TRemoteFile::GetIsParentDirectory
(006D1A14) TRemoteFile::GetFullFileName
(007790C8) TTerminal::CalculateFileSize
(00776AF9) TTerminal::ProcessFiles
(00779526) TTerminal::CalculateFilesSize
(00785271) TTerminal::CopyToLocal
(0001156D) TCustomScpExplorerForm::TemporarilyDownloadFiles
(0000994E) TCustomS

The problem happens rarely. I cannot reproduce it.

<Try to describe precise steps that lead to the problem (where do you click, what keys do you press, what do you see, etc.)>
<If relevant, consider attaching a session log file or a screenshot)>

WinSCP 5.9

Error message:
Invalid access to memory.

Stack trace:
(003E5CA4) System::UStrEqual
(0086216E) C5073_3
(008641D6) ____ExceptionHandler
(00860F90) __ExceptionHandler
(00062496) ntdll.dll
(0000F13E) ntdll.dll.KiUserExceptionDispatcher
(006CCF8D) TRemoteFile::GetIsParentDirectory
(006D045C) TRemoteFile::GetFullFileName
(00777A7C) TTerminal::CalculateFileSize
(007754AD) TTerminal::ProcessFiles
(00777EDA) TTerminal::CalculateFilesSize
(00783C25) TTerminal::CopyToLocal
(0000DD8A) TCustomScpExplorerForm::ExecuteFileOperation
(0000E9E4) TCustomScpExplorerForm::ExecuteFileOperation
(0000EA6B) TCustomScpExplorerForm::ExecuteFileOperationCommand
(0000EBF2) TCustomScpExplorerForm::ExecuteCopyOperationCommand
(00037546) TNonVisualDataModule::ExplorerActionsExecute
(003B4768) System::Actions::TContainedActionList::ExecuteAction
(0019B96D) Vcl::Actnlist::TCustomAction::Execute
(0037264C) System::Classes::_18201
(000152F7) USER32.dll
(00015D35) USER32.dll
(000167CE) USER32.dll
(00016895) USER32.dll.DispatchMessageW

(003E5CA4) System::UStrEqual
(0086216E) C5073_3
(008641D6) ____ExceptionHandler
(00860F90) __ExceptionHandler
(00062496) ntdll.dll
(0000F13E) ntdll.dll.KiUserExceptionDispatcher
(006CCF8D) TRemoteFile::GetIsParentDirectory
(006D045C) TRemoteFile::GetFullFileName
(00777A7C) TTerminal::CalculateFileSize
(007754AD) TTerminal::ProcessFiles
(00777EDA) TTerminal::CalculateFilesSize
(00783C25) TTerminal::CopyToLocal
(0000DD8A) TCustomScpExplorerForm::ExecuteFileOpe

The problem happens rarely. I cannot reproduce it.