When you right click on mouse button.

WinSCP 5.17.8

Error message:
Invalid access to memory.substring(84062)

Stack trace:
(00F78953) C3651_0
(00F89E6E) C5296_0
(00F8BED6) ____ExceptionHandler
(00F88C90) __ExceptionHandler
(000861AF) [771471

Status: all_insignificant

The problem happens rarely. I cannot reproduce it.