<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 6.5.5

Error message:
Invalid access to memory.

Stack trace:
(00C24BC0) System::AnsiStringT<65535> * System::AnsiStringT<65535>::AnsiStringT<65535>
(00EF74C2)
(00EF953A)
(00EF62E4) __ExceptionHandler
(000B332F) ntdll.dll
(0007A56A) ntdll.dll.KiUserExceptionDispatcher
(00C24CA5) UnMungeStr
(00C2660A) THierarchicalStorage::GetCurrentSubKey
(00C26714) THierarchicalStorage::MungingKeyName
(00C268A8) THierarchicalStorage::MungeKeyName
(00C2700A) THierarchicalStorage::OpenSubKey
(00C3E2C2) _open_regkey_fn_winscp
(00BA8476) _retrieve_host_key
(00C9E0D6) TSecureShell::RetrieveHostKey
(00CA336D) TSecureShell::HaveHostKey
(00C3DDD5) _have_ssh_host_key
(00B8CEA5) _ssh2_transport_pop
(00B8E196) _ssh2transport_finalise_exhash
(00B8AA1C) C4444_0
(00B97096) _ssh_verstring_new
(00B7DBD1) _ssh_ppl_new_prompts
(00B377B6) C4305_0
(00B378A9) _run_toplevel_callbacks
(00C9D3B1) TSecureShell::EventSelectLoop
(00C9C2C3) TSecureShell::WaitForData
(00C96BE5) TSecureShell::Init
(00C96526) TSecureShell::Open
(00CF1B55) TSFTPFileSystem::Open
(00D3499B) TTerminal::Reopen
(00D370C1) TTerminal::QueryReopen
(00D29934) TRobustOperationLoop::TryReopen
(00D3C6C9) TTerminal::CustomReadDirectory
(00D3B98D) TTerminal::ReadDirectory
(00D36C77) TTerminal::ReactOnCommand
(00BB5269) TUnixDirView::ChangeDirectory
(00BB3399) TUnixDirView::ExecuteFile
(0050762A) Customdirview::TCustomDirView::DoExecute
(0050825D) Customdirview::TCustomDirView::WMLButtonDblClk
(003A356D) Vcl::Controls::TWinControl::WndProc
(0041B07F) Vcl::Comctrls::TCustomListView::WndProc
(00204C90) System::Classes::_18364
(00038981) USER32.dll
(00026AC8) USER32.dll
(00026235) USER32.dll.CallWindowProcW
(000829E4) COMCTL32.DLL
(00082A83) COMCTL32.DLL
(000827FC) COMCTL32.DLL
(00038981) USER32.dll
(00026AC8) USER32.dll
(00026235) USER32.dll.CallWindowProcW
(00539473) Dragdrop::TDragDrop::WndMethod
(00204C90) System::Classes::_18364
(00038981) USER32.dll
(00026AC8) USER32.dll
(00025F55) USER32.dll
(00025ABB) USER32.dll.DispatchMessageW

(00EF7B2D)
(00EF9107)
(00C96CF7) TSecureShell::Init
(00C96526) TSecureShell::Open
(00CF1B55) TSFTPFileSystem::Open
(00D3499B) TTerminal::Reopen
(00D370C1) TTerminal::QueryReopen
(00D29934) TRobustOperationLoop::TryReopen
(00D3C6C9) TTerminal::CustomReadDirectory
(00D3B98D) TTerminal::ReadDirectory
(00D36C77) TTerminal::ReactOnCommand
(00BB5269) TUnixDirView::ChangeDirectory
(00BB3399) TUnixDirView::ExecuteFile
(0050762A) Customdirview::TCustomDirView::DoExecute
(0050825D) Customdirview::TCustomDirView::WMLButtonDblClk
(003A356D) Vcl::Controls::TWinControl::WndProc
(0041B07F) Vcl::Comctrls::TCustomListView::WndProc
(00204C90) System::Classes::_18364
(00038981) USER32.dll
(00026AC8) USER32.dll
(00026235) USER32.dll.CallWindowProcW
(000829E4) COMCTL32.DLL
(00082A83) COMCTL32.DLL
(000827FC) COMCTL32.DLL
(00038981) USER32.dll
(00026AC8) USER32.dll
(0002623

The problem happens rarely. I cannot reproduce it.