<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 5.7.6

Error message:
External exception C0000006

Stack trace:
(00016D43) IMM32.DLL
(0007C54F) ntdll.dll
(00069D8A) ntdll.dll.KiUserExceptionDispatcher
(00086C35) USER32.dll.WINNLSEnableIME
(002255AB) Vcl::Controls::Win32NLSEnableIME
(0021D428) Vcl::Controls::TWinControl::WMKillFocus
(0021C315) Vcl::Controls::TWinControl::WndProc
(00184D80) Vcl::Stdctrls::TButtonControl::WndProc
(003402A8) System::Classes::_18201
(00033921) USER32.dll
(0001378B) USER32.dll
(0001336B) USER32.dll
(0001A177) USER32.dll
(00069D64) ntdll.dll.KiUserCallbackDispatcher
(001D933B) Vcl::Forms::TCustomForm::DestroyWindowHandle
(00004D85) Vcl::Forms::TForm::~TForm
(0075BD6B) TLoginDialog::~TLoginDialog
(0075B9EF) DoLoginDialog
(000A5953) GetLoginData
(000A948C) Execute
(00002462) wWinMain
(007E2811) C4667_0
(00058E52) ntdll.dll
(00058E1A) ntdll.dll

The problem happens rarely. I cannot reproduce it.