<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 5.9.5

Error message:
Naruszenie ochrony pamęci.

Stack trace:
(00737D0E) TSFTPFileSystem::Idle
(0086700E) C5073_3
(00869076) ____ExceptionHandler
(00865E30) __ExceptionHandler
(000624F6) ntdll.dll
(0000F13E) ntdll.dll.KiUserExceptionDispatcher
(0008EEF0) TTerminalManager::Idle
(00016C9F) TCustomScpExplorerForm::Idle
(0003D057) TNonVisualDataModule::DoIdle
(0003D014) TNonVisualDataModule::SessionIdleTimerTimer
(0037326C) System::Classes::_18201
(000152F7) USER32.dll
(00015D35) USER32.dll
(000167BF) USER32.dll
(00016885) USER32.dll.DispatchMessageW

(00867679) C5073_3
(00868C53) _ReThrowException
(0008EF35) TTerminalManager::Idle
(00016C9F) TCustomScpExplorerForm::Idle
(0003D057) TNonVisualDataModule::DoIdle
(0003D014) TNonVisualDataModule::SessionIdleTimerTimer
(0037326C) System::Classes::_18201
(000152F7) USER32.dll
(00015D35) USER32.dll
(000167BF) USER32.dll
(00016885) USER32.dll.DispatchMessageW

The problem happens rarely. I cannot reproduce it.