Invalid access to memory - _mp_mul_into

<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 6.5.3

Error message:
Invalid access to memory.

Stack trace:
(00B62253) _mp_mul_into
(00EF5B8E)
(00EF7C06)
(00EF49B0) __ExceptionHandler
(000B320F) ntdll.dll
(0007A66A) ntdll.dll.KiUserExceptionDispatcher
(00B62C6C) _monty_mul_into
(00B62CDE) _monty_mul

(00EF61F9)
(00EF77D3)
(000CE65E) TTerminalManager::Idle
(0001CC46) TCustomScpExplorerForm::Idle
(00056D7B) TNonVisualDataModule::DoIdle
(00056D38) TNonVisualDataModule::SessionIdleTimerTimer
(00204B78) System::Classes::_18364
(00038A71) USER32.dll
(00026BE8) USER32.dll
(00026075) USER32.dll
(00025BDB) USER32.dll.DispatchMessageW

The problem happens rarely. I cannot reproduce it.