This bug has been added to the tracker:
https://winscp.net/tracker/1304

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

Copied a file to ftp server, then canceled the transfer but the transfer wouldn't stop.
WinSCP 5.7.1

Error message:
Invalid access to memory.

Stack trace:
[0142F5FF] Vcl::Controls::TWinControl::HandleNeeded
[7727023E] ntdll.dll.ZwWow64CallFunction64
[772709BA] ntdll.dll.KiUserExceptionDispatcher
[0142BE2C] Vcl::Controls::TWinControl::MainWndProc
[01550708] System::Classes::_18201
[74E67831] USER32.DLL.CallNextHookEx
[74E6930A] USER32.DLL
[74E67BE8] USER32.DLL
[74E67D7F] USER32.DLL
[77270994] ntdll.dll.KiUserCallbackDispatcher
[74E6A48E] USER32.DLL
(00000812) UXTHEME.DLL.[73EF1812]
(000007A8) UXTHEME.DLL.[73EF17A8]
(00000DDA) UXTHEME.DLL.[73EF1DDA]
[74E6A424] USER32.DLL.GetGUIThreadInfo
[74E67831] USER32.DLL.CallNextHookEx
[74E67A95] USER32.DLL
[74E6CE6C] USER32.DLL.CallWindowProcW
[0142C8F6] Vcl::Controls::TWinControl::DefaultHandler
[0142C7E5] Vcl::Controls::TWinControl::WndProc
[013E7588] Vcl::Forms::TCustomForm::WndProc
[0199B255] Vclcommon::C2516_0
[0142BE2C] Vcl::Controls::TWinControl::MainWndProc
[01550708] System::Classes::_18201
[74E67831] USER32.DLL.CallNextHookEx
[74E67A95] USER32.DLL
[74E69889] USER32.DLL
[74E698EC] USER32.DLL.DispatchMessageW