Invalid access to memory

<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 5.21.8

Error message:
Invalid access to memory.

Stack trace:
(00D045D8)
(00087AFF) ntdll.dll
(00073FD1) ntdll.dll.KiUserExceptionDispatcher
(0001A994)
(0001AA3E)
(002824BD)
(00281B04)
(003C9C40)
(0003FE89) USER32.dll
(00036E55) USER32.dll
(00036532) USER32.dll
(000346D6) USER32.dll.CallWindowProcW
(004A18FF)
(003C9C40)
(0003FE89) USER32.dll
(00036E55) USER32.dll
(00034BC5) USER32.dll
(0003498B) USER32.dll.DispatchMessageW

Status: unknown_rel_addr

The problem happens rarely. I cannot reproduce it.