<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 5.9.1

Error message:
Invalid access to memory.

Stack trace:
(006C22E0)
(00082F9F) ntdll.dll
(0007036A) ntdll.dll.KiUserExceptionDispatcher
(000171CE)
(00017270)
(0024A4C0)
(0024A558)
(0024A73D)
(0024EA59)
(0024E0A0)
(00372990)
(0002CAE1) USER32.dll
(0000DABB) USER32.dll
(0000CF32) USER32.dll.CallWindowProcW
(0042BE5F)
(00372990)
(0002CAE1) USER32.dll
(0000DABB) USER32.dll
(0000D1F4) USER32.dll
(0000CFBB) USER32.dll.DispatchMessageW

Status: unknown_rel_addr

The problem happens rarely. I cannot reproduce it.