<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 5.15.5

Error message:
Invalid access to memory.

Stack trace:
(FFFFF010)WinSCP.exe
(0006248E) ntdll.dll
(0000F15E) ntdll.dll.KiUserExceptionDispatcher
(0081AB3D) TLoginDialog::UpdateControls
(0082479A) TMessageForm::CreateButton
(008194EA) TLoginDialog::InitControls
(006D09D2) TFTPFileSystem::HandleAsynchRequestVerifyCertificate
(00447311) __stdcall Tb2acc::TTBItemViewerAccObject::accNavigate
(0000A1B7) TCustomScpExplorerForm::LocalCustomCommandPure
(003A3E44) System::Masks::TMask::MatchesMaskStates
(000152F7) USER32.dll
(00015D35) USER32.dll
(000167C2) USER32.dll
(00016885) USER32.dll.DispatchMessageW

The problem happens rarely. I cannot reproduce it.