The remote server to which I was trying to connect was down.
Retried a few times with WinSCP then pressed "Cancel"
While I was trying to connect there was also a small dialog box saying
Drive C: is not ready
maybe because I connected to the computer running WinSCP via a new Remote Desktop connection from a different machine than when I started WinSCP.
The drive C was working well when I checked.

WinSCP 6.1.1

Error message:
Invalid access to memory.

Stack trace:
(0029466B) System::DoneExcept
(00087A7F) ntdll.dll
(000740D1) ntdll.dll.KiUserExceptionDispatcher
(00214D98) System::Classes::_18201
(000401A9) USER32.dll
(00037165) USER32.dll
(00034EC5) USER32.dll
(00023B10) USER32.dll.DialogBoxIndirectParamAorW
(000239AF) USER32.dll.DialogBoxIndirectParamAorW
(000238CD) USER32.dll.DialogBoxIndirectParamAorW
(00023886) USER32.dll.DialogBoxIndirectParamW
(0018C6A2) COMCTL32.DLL
(000D7B69) COMCTL32.DLL
(000D8641) COMCTL32.DLL.TaskDialogIndirect
(001CED61) Winapi::Commctrl::TaskDialogIndirect
(003DCF15) Vcl::Dialogs::TCustomTaskDialog::DoExecute
(003DD21D) Vcl::Dialogs::TCustomTaskDialog::Execute
(003DEB16) Vcl::Dialogs::_16828
(003DE4A2) Vcl::Dialogs::MessageDlgPosHelp
(004902B8) Driveview::TDriveView::CanChange
(0042C30A) Vcl::Comctrls::TCustomTreeView::CNNotify
(0047FB11) Customdriveview::TCustomDriveView::CNNotify
(003F62DD) Vcl::Controls::TWinControl::WndProc
(0042C62E) Vcl::Comctrls::TCustomTreeView::WndProc
(003F6443) Vcl::Controls::_17076
(003F62DD) Vcl::Controls::TWinControl::WndProc
(003F5924) Vcl::Controls::TWinControl::MainWndProc
(00214D98) System::Classes::_18201
(000401A9) USER32.dll
(00037165) USER32.dll
(00036D55) USER32.dll
(0003A92D) USER32.dll
(0007408B) ntdll.dll.KiUserCallbackDispatcher
(00034B7A) USER32.dll.SendMessageW
(00068881) COMCTL32.DLL
(000288A2) COMCTL32.DLL
(00028695) COMCTL32.DLL
(0003EC68) COMCTL32.DLL
(000401A9) USER32.dll
(00037165) USER32.dll
(00036832) USER32.dll
(000349D6) USER32.dll.CallWindowProcW
(003F63EE) Vcl::Controls::TWinControl::DefaultHandler
(003F62DD) Vcl::Controls::TWinControl::WndProc
(0042C62E) Vcl::Comctrls::TCustomTreeView::WndProc
(00214D98) System::Classes::_18201
(000401A9) USER32.dll
(00037165) USER32.dll
(00036832) USER32.dll
(000349D6) USER32.dll.CallWindowProcW
(004AFCAF) Dragdrop::TDragDrop::WndMethod
(00214D98) System::Classes::_18201
(000401A9) USER32.dll
(00037165) USER32.dll
(00036D55) USER32.dll
(0003A92D) USER32.dll
(0007408B) ntdll.dll.KiUserCallbackDispatcher
(00034B7A) USER32.dll.SendMessageW
(001CED07) Winapi::Commctrl::TreeView_SelectItem
(0049260F) Driveview::TDriveView::CallBackValidateDir
(00480351) Customdriveview::TCustomDriveView::IterateSubTree
(00492ADE) Driveview::TDriveView::ValidateDirectoryEx
(0047FFF9) Customdriveview::TCustomDriveView::ValidateDirectory
(003CCDAF) Vcl::Extctrls::TTimer::Timer
(00214D98) System::Classes::_18201
(000401A9) USER32.dll
(00037165) USER32.dll
(00034EC5) USER32.dll
(00023B10) USER32.dll.DialogBoxIndirectParamAorW
(000239AF) USER32.dll.DialogBoxIndirectParamAorW
(000238CD) USER32.dll.DialogBoxIndirectParamAorW
(00023886) USER32.dll.DialogBoxIndirectParamW
(0018C6A2) COMCTL32.DLL
(000D7B69) COMCTL32.DLL
(000D8641) COMCTL32.DLL.TaskDialogIndirect
(001CEDA6) Winapi::Commctrl::TaskDialogIndirect
(003DCF15) Vcl::Dialogs::TCustomTas

The problem happens rarely. I cannot reproduce it.