<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 idle for about 24 hours, session invalid after about 1 hour of idleness.

WinSCP 5.7.7

Error message:
Naruszenie ochrony pamęci.

Stack trace:
(00340638) System::Classes::MakeObjectInstance
(007E08AA) C4799_0
(007E2912) ____ExceptionHandler
(007DF88C) __ExceptionHandler
(00062496) ntdll.dll
(0000F13E) ntdll.dll.KiUserExceptionDispatcher
(003407DE) System::Classes::AllocateHWnd
(001F2F88) Vcl::Extctrls::TTimer::TTimer
(0009FA7B) MoreMessageDialog
(000A1160) ExceptionMessageDialog
(000A1411) FatalExceptionMessageDialog
(00083536) ShowExtendedExceptionEx
(0001AB28) TCustomScpExplorerForm::ShowExtendedException
(00077BDD) TTerminalManager::TerminalShowExtendedException
(0070FC38) TTerminal::ShowExtendedException
(00076436) TTerminalManager::ReconnectActiveTerminal
(000835BA) ShowExtendedExceptionEx
(0001AB28) TCustomScpExplorerForm::ShowExtendedException
(00077BDD) TTerminalManager::TerminalShowExtendedException
(0070FC38) TTerminal::ShowExtendedException
(00076742) TTerminalManager::SetActiveTerminal
(00019E50) TCustomScpExplorerForm::SessionsPageControlChange
(0021C609) Vcl::Controls::TWinControl::WndProc
(00217700) Vcl::Controls::TControl::Perform
(0021C76F) Vcl::Controls::_17076
(0021C609) Vcl::Controls::TWinControl::WndProc
(001D73AC) Vcl::Forms::TCustomForm::WndProc
(0078E0D1) Vclcommon::C2516_0
(0021BC50) Vcl::Controls::TWinControl::MainWndProc
(0034059C) System::Classes::_18201
(000152F7) USER32.dll
(00015D35) USER32.dll
(00015DE8) USER32.dll
(00015E49) USER32.dll
(0000F117) ntdll.dll.KiUserCallbackDispatcher
(000186D0)

The problem happens rarely. I cannot reproduce it.