<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 5.9.2

Error message:
Zugriffsverletzung - ungültiger Zugriff auf den Speicher.

Stack trace:
(00649081) TFileOperationProgressType::SetFile
(00863846) D1514_1
(008658AE) D1566_1
(00862668) D1482_1
(0008147F) ntdll.dll
(0006F02A) ntdll.dll.KiUserExceptionDispatcher
(00646556) TFileCustomCommand::Validate
(00646A5B) TFileMasks::TMask * std::_Copy_backward_opt<TFileMasks::TMask *, TFileMasks::TMask *>
(0064A7D4) std::vector<unsigned long, std::allocator<unsigned long> >::_Insert_n
(0064A8A5) std::vector<unsigned long, std::allocator<unsigned long> >::_Insert_n
(0061674D) __tpdsc__ void __closure
(0061C92B) __tpdsc__ TBookmarkList
(0062920D) std::_Tree<std::_Tmap_traits<int, TDateTimeParams, std::less<int>, std::allocator<std::pair<const const int, TDateTimeParams> >, 0> >::_Insert
(006F6A7B) TSFTPFileSystem::SpaceAvailable
(0076D91E) TOpenDirectoryDialog::GetBookmarkList
(006C6A1F) TStoredSessionList::UpdateStaticUsage
(006C19F4) TSessionData::SetAuthKI
(000987A7) __tpdsc__ TScpCommanderConfiguration
(000152C2) KERNEL32.DLL.BaseThreadInitThunk
(0005F717) ntdll.dll
(0005F6DF) ntdll.dll

(00863EB1) D1518_1
(0086548B) _rand_ssleay_meth
(006F6C38) TSFTPFileSystem::SpaceAvailable
(0076D91E) TOpenDirectoryDialog::GetBookmarkList
(006C6A1F) TStoredSessionList::UpdateStaticUsage
(006C19F4) TSessionData::SetAuthKI
(000987A7) __tpdsc__ TScpCommanderConfiguration
(000152C2) KERNEL32.DLL.BaseThreadInitThunk
(0005F717) ntdll.dll%

The problem happens rarely. I cannot reproduce it.