<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 6.3.3

Error message:
Acceso a memoria no válido

Rastreo de la pila:
(002A761F) System::DoneExcept
(000987BF) ntdll.dll
(00077E51) ntdll.dll.KiUserExceptionDispatcher
(002271BC) System::Classes::_18201
(00032451) USER32.dll
(000232A1) USER32.dll
(00021B60) USER32.dll
(000216AB) USER32.dll.DispatchMessageW

(002A720C) System::HandleAnyException
(000987BF) ntdll.dll
(00077E51) ntdll.dll.KiUserExceptionDispatcher
(002271BC) System::Classes::_18201
(00032451) USER32.dll
(000232A1) USER32.dll
(00021B60) USER32.dll
(000216AB) USER32.dll.DispatchMessageW

Status: all_insignificant

The problem happens rarely. I cannot reproduce it.