<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:
(00D244BE) TSFTPFileSystem::Sink
(00EF74C2)
(00EF953A)
(00EF62E4) __ExceptionHandler
(000B332F) ntdll.dll
(0007A56A) ntdll.dll.KiUserExceptionDispatcher
(00D53DDC) TTerminal::DoCopyToLocal
(00D20A5A) TSFTPFileSystem::CopyToLocal
(00D4FEFA) TTerminal::CopyToParallel
(00C49C5F) TParallelTransferQueueItem::DoExecute
(00C486C6) TQueueItem::Execute
(00C476BF) TTerminalItem::ProcessEvent
(00C459F0) TSignalThread::Execute
(000DF53B) Userinterface::C20_0
(00014D47) KERNEL32.DLL.BaseThreadInitThunk
(0006C5D9) ntdll.dll
(0006C55C) ntdll.dll

(00D244BE) TSFTPFileSystem::Sink
(00EF74C2)
(00EF953A)
(00EF62E4) __ExceptionHandler
(000B332F) ntdll.dll
(0007A56A) ntdll.dll.KiUserExceptionDispatcher
(00D53DDC) TTerminal::DoCopyToLocal
(00D20A5A) TSFTPFileSystem::CopyToLocal
(00D4FEFA) TTerminal::CopyToParallel
(00C49C5F) TParallelTransferQueueItem::DoExecute
(00C486C6) TQueueItem::Execute
(00C476BF) TTerminalItem::ProcessEvent
(00C459F0) TSignalThread::Execute
(000DF53B) Userinterface::C20_0
(00014D47) KERNEL32.DLL.BaseThreadInitThunk
(0006C5D9) ntdll.dll
(0006C55C) ntdll.dll

The problem happens rarely. I cannot reproduce it.