What I did:
1. connect to remote server
2. select some folder on remote server and click F6. Accept default settings.
3. Let some files transfer, wait 20 seconds.
4. Click "Proceed in background" symbol
5. Error message "Invalid access to memory" was shown only some seconds afterwards.



WinSCP 5.9.1

Error message:
Invalid access to memory.

Stack trace:
(0032390C) System::Internal::Strhlpr::UnicodeAssign
(00862736) C5073_3
(0086479E) ____ExceptionHandler
(00861558) __ExceptionHandler
(0008137F) ntdll.dll
(0006EF1A) ntdll.dll.KiUserExceptionDispatcher
(006D1306) TRemoteFileList::GetFullDirectory
(006D0DD3) TRemoteFile::GetFullFileName
(007782AC) TTerminal::CalculateFileSize
(00775B25) TTerminal::ProcessFiles
(00778552) TTerminal::CalculateFilesSize
(0078429D) TTerminal::CopyToLocal
(006C5697) TDownloadQueueItem::DoExecute
(006C2896) TTerminalItem::ProcessEvent
(006C0D3C) TSignalThread::Execute
(000986B3) Userinterface::C20_0
(000152C2) KERNEL32.DLL.BaseThreadInitThunk
(0005F607) ntdll.dll
(0005F5CF) ntdll.dll

(0032390C) System::Internal::Strhlpr::UnicodeAssign
(00862736) C5073_3
(0086479E) ____ExceptionHandler
(00861558) __ExceptionHandler
(0008137F) ntdll.dll
(0006EF1A) ntdll.dll.KiUserExceptionDispatcher
(006D1306) TRemoteFileList::GetFullDirectory
(006D0DD3) TRemoteFile::GetFullFileName
(007782AC) TTerminal::CalculateFileSize
(00775B25) TTerminal::ProcessFiles
(00778552) TTerminal::CalculateFilesSize
(0078429D) TTerminal::CopyToLocal
(006C5697) TDownloadQueueItem::DoExecute
(006C2896) TTerminalItem::ProcessEvent
(006C0D3C) TSignalThread::Execute
(000986B3) Userinterface::C20_0
(000152C2) KERNEL32.DLL.BaseThreadInitThunk
(0005F607) ntdll.dll
(0005F5CF) ntdll.dll

(00862DA1) C5073_3
(0086437B) _ReThrowException

The problem happens rarely. I cannot reproduce it.

<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 5.7.7

Error message:
Naruszenie ochrony pamęci.

Stack trace:
(002F14D4)
(007E08AA)
(007E2912)
(007DF88C)
(0008A61F) ntdll.dll
(00077E7A) ntdll.dll.KiUserExceptionDispatcher
(006806A6)
(00680173)
(00718A0D)
(007161B1)
(00718CEC)
(007241D5)
(00675077)
(00672277)
(00670728)
(0008269F)
(000128F2) KERNEL32.DLL.BaseThreadInitThunk
(00064DE1) ntdll.dll
(00064DA9) ntdll.dll

(002F14D4)
(007E08AA)
(007E2912)
(007DF88C)
(0008A61F) ntdll.dll
(00077E7A) ntdll.dll.KiUserExceptionDispatcher
(006806A6)
(00680173)
(00718A0D)
(007161B1)
(00718CEC)
(007241D5)
(00675077)
(00672277)
(00670728)
(0008269F)
(000128F2) KERNEL32.DLL.BaseThreadInitThunk
(00064DE1) ntdll.dll
(00064DA9) ntdll.dll

(007E0F15)
(007E24EF)
(007163B8)
(00718CEC)
(007241D5)
(00675077)
(00672277)
(00670728)
(0008269F)
(000128F2) KERNEL32.DLL.BaseThreadInitThunk
(00064DE1) ntdll.dll
(00064DA9) ntdll.dll

Status: unknown_rel_addr

The problem happens rarely. I cannot reproduce it.