<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 5.17.5

Error message:
Invalid access to memory.

Stack trace:
(00C2A670) _base64_encode_atom
(00F87C7E) C5296_3
(00F89CE6) ____ExceptionHandler
(00F86AA0) __ExceptionHandler
(00088F4F) ntdll.dll
(000743E1) ntdll.dll.KiUserExceptionDispatcher
(00C1F91D) _ssh2_fingerprint_blob
(00C1FA7F) _ssh2_fingerprint
(00BF02A5) _ssh2kex_coroutine
(00BF2444) _ssh2transport_finalise_exhash
(00BFF139) _ssh_ppl_free
(00BCDC1E) C2603_0
(00BCDD11) _run_toplevel_callbacks
(00D076F8) TSecureShell::EventSelectLoop
(00D065DB) TSecureShell::WaitForData
(00D01599) TSecureShell::Init
(00D00ED9) TSecureShell::Open
(00D890D1) TTerminal::Open
(00CC0A6B) TTerminalThread::TerminalOpenEvent
(00CBB9C8) TSignalThread::Execute
(000BC24F) Userinterface::C20_0
(00015357) KERNEL32.DLL.BaseThreadInitThunk
(00067962) ntdll.dll.RtlGetAppContainerNamedObjectPath
(0006792F) ntdll.dll.RtlGetAppContainerNamedObjectPath

(00F882E9) C5296_3
(00F898C3) _ReThrowException
(00D016AB) TSecureShell::Init
(00D00ED9) TSecureShell::Open
(00D890D1) TTerminal::Open
(00CC0A6B) TTerminalThread::TerminalOpenEvent
(00CBB9C8) TSignalThread::Execute
(000BC24F) Userinterface::C20_0
(00015357) KERNEL32.DLL.BaseThreadInitThunk
(00067962) ntdll.dll.RtlGetAppContainerNamedObjectPath
(0006792F) ntdll.dll.RtlGetAppContainerNamedObjectPath

(00F882E9) C5296_3
(00F898C3) _ReThrowException
(00D01746) TSecureShell::Init
(00D00ED9) TSecureShell::Open
(00D890D1) TTerminal::Open
(00CC0A6B) TTerminalThread::TerminalOpenEvent
(00CBB9C8) TSignalThread::Execute
(000BC24F) Userinterface::C20_0
(00015357) KERNEL32.DLL.BaseThreadInitThunk
(00067962) ntdll.dll.RtlGetAppContainerNamedObjectPath
(0006792F) ntdll.dll.RtlGetAppContainerNamedObjectPath

(00F882E9) C5296_3
(00F898C3) _ReThrowException
(00D00FC7) TSecureShell::Open
(00D890D1) TTerminal::Open
(00CC0A6B) TTerminalThread::TerminalOpenEvent
(00CBB9C8) TSignalThread::Execute
(000BC24F) Userinterface::C20_0
(00015357) KERNEL32.DLL.BaseThreadInitThunk
(00067962) ntdll.dll.RtlGetAppContainerNamedObjectPath
(0006792F) ntdll.dll.RtlGetAppContainerNamedObjectPath

(00F882E9) C5296_3
(00F898C3) _ReThrowException
(00D89279) TTerminal::Open
(00CC0A6B) TTerminalThread::TerminalOpenEvent
(00CBB9C8) TSignalThread::Execute
(000BC24F) Userinterface::C20_0
(00015357) KERNEL32.DLL.BaseThreadInitThunk
(00067962) ntdll.dll.RtlGetAppContainerNamedObjectPath
(0006792F) ntdll.dll.RtlGetAppContainerNamedObjectPath

(00F882E9) C5296_3
(00F898C3) _ReThrowException
(00D898A2) TTerminal::Open
(00CC0A6B) TTerminalThread::TerminalOpenEvent
(00CBB9C8) TSignalThread::Execute
(000BC24F) Userinterface::C20_0
(00015357) KERNEL32.DLL.BaseThreadInitThunk
(00067962) ntdll.dll.RtlGetAppContainerNamedObjectPath
(0006792F) ntdll.dll.RtlGetAppContainerNamedObjectPath

The problem happens rarely. I cannot reproduce it.