Thanks for your report.
I have sent you an email with a debug version of WinSCP to address you have used to register on this forum.

<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 5.6.3

Error message:
Invalid access to memory.

Stack trace:
[01978D93] TFTPFileSystem::HandleCapabilities
[01AF6B82] C4727_3
[01AF8BEA] ____ExceptionHandler
[01AF5B64] __ExceptionHandler
[77DBB466] ntdll.dll
[77D7012E] ntdll.dll.KiUserExceptionDispatcher
[01973A28] TFTPFileSystem::ProcessMessage
[01973A45] TFTPFileSystem::DiscardMessages
[01A1F9EB] TTerminal::~TTerminal
[013953C5] TManagedTerminal::~TManagedTerminal
[013966D8] TTerminalManager::FreeTerminal
[01395D6F] TTerminalManager::FreeActiveTerminal
[01396482] TTerminalManager::ReconnectActiveTerminal
[013A35C2] ShowExtendedExceptionEx
[0133B580] TCustomScpExplorerForm::ShowExtendedException
[01397BB9] TTerminalManager::TerminalShowExtendedException
[01A23800] TTerminal::ShowExtendedException
[0165F9BC] System::Classes::_18201
[769662F7] USER32.dll
[76966D35] USER32.dll
[769677BF] USER32.dll
[76967885] USER32.dll.DispatchMessageW

[01AF71ED] C4727_3
[01AF87C7] _ReThrowException
[013964A7] TTerminalManager::ReconnectActiveTerminal
[013A35C2] ShowExtendedExceptionEx
[0133B580] TCustomScpExplorerForm::ShowExtendedException
[01397BB9] TTerminalManager::TerminalShowExtendedException
[01A23800] TTerminal::ShowExtendedException
[0165F9BC] System::Classes::_18201
[769662F7] USER32.dll
[76966D35] USER32.dll
[769677BF] USER32.dll
[76967885] USER32.dll.DispatchMessageW