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

In WinSCP, the error occurs when righ-clicking on the title bar of a folder tree pane and selecting "Add Path to Bookmarks". It happens for the local as well as the remote path.

WinSCP 5.15.9

Error message:
Invalid access to memory.

Stack trace:
(00894978) TBookmarkList * GetBookmarkObject<TBookmarkList>
(000B30C2)ntdll.dll
(000B366C)ntdll.dll
(000B39AF)ntdll.dll
(00894958) TOpenDirectoryDialog::GetBookmarkList
(00894A6D) TOpenDirectoryDialog::AddAsBookmark
(00894806) TOpenDirectoryDialog::Execute
(008938EE) DoOpenDirectoryDialog
(0001EF2A) TCustomScpExplorerForm::DoOpenDirectoryDialog
(000504DC) TScpCommanderForm::DoOpenDirectoryDialog
(0001EA92) TCustomScpExplorerForm::AddBookmark
(003E78A0) System::Actions::TContainedActionList::ExecuteAction
(001C6489) Vcl::Actnlist::TCustomAction::Execute
(003A3E7C) System::Classes::_18201
(000A605A)user32.dll
(000A66DC)user32.dll
(000A890E)user32.dll
(0006D0ED)user32.dll

I know how to reproduce the problem or the problem happens frequently enough. I wish to be contacted by the WinSCP team to help resolving the problem.