<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 6.3.5

Error message:
Invalid access to memory.

Stack trace:
(00B3A624) TFileOperationProgressType::GetCPSLimit
(00098D8F) ntdll.dll
(000781B1) ntdll.dll.KiUserExceptionDispatcher
(00B3A62D) TFileOperationProgressType::GetCPSLimit
(00B3A62D) TFileOperationProgressType::GetCPSLimit
(00B7C5FC) TQueueItem::GetCPSLimit
(00B7C995) TQueueItemProxy::GetCPSLimit

(00B3A624) TFileOperationProgressType::GetCPSLimit
(00098D8F) ntdll.dll
(000781B1) ntdll.dll.KiUserExceptionDispatcher
(00B3A62D) TFileOperationProgressType::GetCPSLimit
(00B3A62D) TFileOperationProgressType::GetCPSLimit
(00B7C5FC) TQueueItem::GetCPSLimit
(00B7C995) TQueueItemProxy::GetCPSLimit

(00B3A624) TFileOperationProgressType::GetCPSLimit
(00098D8F) ntdll.dll
(000781B1) ntdll.dll.KiUserExceptionDispatcher
(00B3A62D) TFileOperationProgressType::GetCPSLimit
(00B3A62D) TFileOperationProgressType::GetCPSLimit
(00B7C5FC) TQueueItem::GetCPSLimit
(00B7C995) TQueueItemProxy::GetCPSLimit

The problem happens rarely. I cannot reproduce it.