Thanks for your report.

This bug is tracked here:
https://winscp.net/tracker/1924

Right Click on panel to add a new file. Right Click Context menu doesn't even appear just instantly get this error

WinSCP 5.17.8

Error message:
Invalid access to memory.substring(75438)

Stack trace:
(00F78953) C3651_0
(00F89E6E) C5296_0
(00F8BED6) ____ExceptionHandler
(00F88C90) __ExceptionHandler
(000861AF) [772071

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.