<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 5.15.1

Error message:
Invalid access to memory.

Stack trace:
(002AA06_) Vcl::Comctrls::TCommonCalendar::CheckValidDate
(0008247F) ntdll.dll
(0006FFFA) ntdll.dll.KiUserExceptionDispatcher
(0044A225) __tpdsc__ Tb2dock::TTBInsertRemoveEvent
(0044A225) __tpdsc__ Tb2dock::TTBInsertRemoveEvent
(0044A2B4) __tpdsc__ Tb2dock::TTBRequestDockEvent
(0045CA0E) __tpdsc__ Tb2item::ITBItems
(00449FA9) Tb2common::GetInputLocaleCodePage
(0024F293) Vcl::Controls::TCustomHintShowHideThread::
(003A346C) __tpdsc__ System::Masks::TMask::TMaskSet
(0002C311) USER32.dll
(0000D8D5) USER32.dll
(0000D22F) USER32.dll
(0000CFDB) USER32.dll.DispatchMessageW

The problem happens rarely. I cannot reproduce it.