<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 5.13.4

Error message:
Invalid access to memory.

Stack trace:
(0063CF6B) _pfl_terminate
(008BDC8E) C5104_3
(008BFCF6) ____ExceptionHandler
(008BCAB0) __ExceptionHandler
(0007DABF) ntdll.dll
(0006B6E1) ntdll.dll.KiUserExceptionDispatcher
(0064390F) C2484_0
(00650B45) _sshfwd_x11_is_local
(007386C0) TSecureShell::FreeBackend
(007386A3) TSecureShell::SetActive
(007B7885) TTerminal::CloseTunnel
(007B779F) TTerminal::OpenTunnel
(007B5B1A) TTerminal::Open
(006F2A43) TTerminalThread::TerminalOpenEvent
(006EDA58) TSignalThread::Execute
(000A3B0B) Userinterface::C20_0
(00017482) KERNEL32.DLL.BaseThreadInitThunk
(0006202A) ntdll.dll.RtlValidSecurityDescriptor
(00061FF5) ntdll.dll.RtlValidSecurityDescriptor

(008BE2F9) C5104_3
(008BF8D3) _ReThrowException
(007B66E1) TTerminal::Open
(006F2A43) TTerminalThread::TerminalOpenEvent
(006EDA58) TSignalThread::Execute
(000A3B0B) Userinterface::C20_0
(00017482) KERNEL32.DLL.BaseThreadInitThunk
(0006202A) ntdll.dll.RtlValidSecurityDescriptor
(00061FF5) ntdll.dll.RtlValidSecurityDescriptor

The problem happens rarely. I cannot reproduce it.