<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 6.1.2

Error message:
Invalid access to memory.

Stack trace:
(001C439B) System::Variants::_16454
(001C4450) System::Variants::VarResultCheck
(001C09C6) System::Internal::Varhlpr::VariantGetElement
(00DBFDAC) TSFTPFileSystem::RemoveReservation
(00DC1AEE) TSFTPFileSystem::UnreserveResponse
(00DBB9C1) TSFTPFileSystem::Close
(000BE50A) TTerminalManager::FreeAll
(000BD1E3) TTerminalManager::~TTerminalManager
(000BCCAC) TTerminalManager::DestroyInstance
(001179EC) Execute
(000027EF) wWinMain
(01001DFD) C5951_0
(0006AD29) ntdll.dll
(0006ACAA) ntdll.dll

(002917D4) System::SysFreeMem
(01003DEA) C6083_0
(01005E52) ____ExceptionHandler
(01002C0C) __ExceptionHandler
(0009812F) ntdll.dll
(00077D31) ntdll.dll.KiUserExceptionDispatcher
(00FF1CEF) ___org_free
(00FDD9D9) ___org_delete
(00FDD9AC) ___org_deletea
(00DBC857) TSFTPFileSystem::ResetConnection
(00DBB755) TSFTPFileSystem::~TSFTPFileSystem
(00DF514B) TTerminal::~TTerminal
(000BCC04) TManagedTerminal::~TManagedTerminal
(000BE787) TTerminalManager::FreeTerminal
(000BE50A) TTerminalManager::FreeAll
(000BD1E3) TTerminalManager::~TTerminalManager
(000BCCAC) TTerminalManager::DestroyInstance
(001179EC) Execute
(000027EF) wWinMain
(01001DFD) C5951_0
(0006AD29) ntdll.dll
(0006ACAA) ntdll.dll

The problem happens rarely. I cannot reproduce it.