<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 5.11.3

Error message:
External exception 80000004

Stack trace:
(00009B85) COMCTL32.DLL
(0003EF0E) ntdll.dll.ZwWow64CallFunction64
(0003F68A) ntdll.dll.KiUserExceptionDispatcher
(00009D32) COMCTL32.DLL
(00007E6F) USER32.DLL.CallNextHookEx
(000080CC) USER32.DLL
(00013D90) USER32.DLL.CallWindowProcW
(001BDF1A) Vcl::Stdctrls::TCustomCombo::ComboWndProc
(001BF685) Vcl::Stdctrls::TCustomComboBox::ComboWndProc
(001BDB07) Vcl::Stdctrls::TCustomCombo::EditWndProc
(00007E6F) USER32.DLL.CallNextHookEx
(00009E98) USER32.DLL
(00008327) USER32.DLL
(00008527) USER32.DLL
(0003F664) ntdll.dll.KiUserCallbackDispatcher
(00007389) COMCTL32.DLL
(00009D32) COMCTL32.DLL
(00007E6F) USER32.DLL.CallNextHookEx
(000080CC) USER32.DLL
(00013D90) USER32.DLL.CallWindowProcW
(001BDF1A) Vcl::Stdctrls::TCustomCombo::ComboWndProc
(001BF685) Vcl::Stdctrls::TCustomComboBox::ComboWndProc
(001BDB07) Vcl::Stdctrls::TCustomCombo::EditWndProc
(00007E6F) USER32.DLL.CallNextHookEx
(00009E98) USER32.DLL
(00008327) USER32.DLL
(00008527) USER32.DLL
(0003F664) ntdll.dll.KiUserCallbackDispatcher
(000096DB) USER32.DLL.DispatchMessageW

The problem happens rarely. I cannot reproduce it.