Invalid access to memory - _portfwd_raw_setup

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WinSCP 5.21.5

Error message:
Invalid access to memory.

Stack trace:
(00C52B0B) _portfwd_raw_setup
(00FDC86E) C5412_0
(00FDE8D6) ____ExceptionHandler
(00FDB690) __ExceptionHandler
(0008792F) ntdll.dll
(00073F81) ntdll.dll.KiUserExceptionDispatcher
(00C52D7F) _portfwd_raw_setup
(00C52E1F) _portfwdmgr_close_all
(00C52E48) _portfwdmgr_free
(00C4B327) _ssh2_connection_new
(00C4A952) _ssh_ppl_free
(00C57AE7) _ssh2_transport_new
(00C4A952) _ssh_ppl_free
(00C56675) _ssh_conn_processed_data
(00C56698) _ssh_conn_processed_data
(00C57110) NONVISUAL.OBJ
(00D4EEA6) TSecureShell::FreeBackend
(00D481C8) TSecureShell::~TSecureShell
(00DD9F39) TTerminal::CloseTunnel
(00DD8EB5) TTerminal::OpenTunnel
(00DD6FB7) TTerminal::Open
(00D062A3) TTerminalThread::TerminalOpenEvent
(00D01110) TSignalThread::Execute
(000C27FB) Userinterface::C20_0
(0001ECC7) KERNEL32.DLL.BaseThreadInitThunk
(00066B1C) ntdll.dll.RtlGetAppContainerNamedObjectPath
(00066AE9) ntdll.dll.RtlGetAppContainerNamedObjectPath

(00FDCED9) C5412_0
(00FDE4B3) _ReThrowException
(00DD7CA5) TTerminal::Open
(00D062A3) TTerminalThread::TerminalOpenEvent
(00D01110) TSignalThread::Execute
(000C27FB) Userinterface::C20_0
(0001ECC7) KERNEL32.DLL.BaseThreadInitThunk
(00066B1C) ntdll.dll.RtlGetAppContainerNamedObjectPath
(00066AE9) ntdll.dll.RtlGetAppContainerNamedObjectPath

The problem happens rarely. I cannot reproduce it.