This issue is being tracked already:
https://winscp.net/tracker/1915

<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 5.17.8

Error message:
Ошибка доступа к памяти.substring(67566)

Трассировка стека:
(00F78960) C3651_0
(00F89E6E) C5296_0
(00F8BED6) ____ExceptionHandler
(00F88C90) __ExceptionHandler
(0005A466) [7706B4

Status: all_insignificant

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.