@scbray: Thanks for your report.
I have sent you an email with a debug version of WinSCP to the address you have used to register on this forum.

<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 5.21.5

Error message:
Error skipping startup message. Your shell is probably incompatible with the application (BASH is recommended).

Invalid access to memory.

Stack trace:
(00C538E2) _is_pfwd
(00FDC86E) C5412_0
(00FDE8D6) ____ExceptionHandler
(00FDB690) __ExceptionHandler
(00087B7F) ntdll.dll
(00074001) ntdll.dll.KiUserExceptionDispatcher
(00CFAD2D) GetSecureShell
(00CFAD6C) _get_callback_set
(00C47910) C2690_0
(00C47A54) _agentf_new
(00C4B94D) _ssh2_connection_new
(00C4CE7C) _ssh2_connection_new
(00C4A961) _ssh_ppl_free
(00C0A02A) C2593_0
(00C0A11D) _run_toplevel_callbacks
(00D502AA) TSecureShell::EventSelectLoop
(00D4F50F) TSecureShell::WaitForData
(00D4C8FA) TSecureShell::Receive
(00D4CB78) TSecureShell::ReceiveLine
(00D2B91E) TSCPFileSystem::ReadCommandOutput
(00D2C054) TSCPFileSystem::ExecCommand
(00D2C103) TSCPFileSystem::ExecCommand
(00D2C614) TSCPFileSystem::SkipStartupMessage
(00D2C2F5) TSCPFileSystem::DoStartup
(00DE0A2D) TTerminal::DoStartup
(00DD7990) TTerminal::Open
(00D02D94) TTerminalItem::ProcessEvent
(00D01110) TSignalThread::Execute
(000C27FB) Userinterface::C20_0
(0001EEF7) KERNEL32.DLL.BaseThreadInitThunk
(00066BBC) ntdll.dll.RtlGetAppContainerNamedObjectPath
(00066B89) ntdll.dll.RtlGetAppContainerNamedObjectPath

I know how to reproduce the problem or the problem happens frequently enough. I wish to be contacted by the WinSCP team to help resolving the problem.