I found the same problem.
It look to come from non conventional characters in my SSH password like "(", "-" ...
Password with only letters and numbers work fine ... but it's not so secure...
So it cannot be the same problem then. The problem @mokaz has occurs before authentication.
Your problem is more likely due to WinSCP 5.9 correctly using UTF-8 for passwords and your server possibly does not. Older versions of WinSCP did not use UTF-8. While that was incorrect, it may have made WinSCP compatible with your server. Though characters like "(" and "-" are ok. They are ascii. Are you sure you do not have any other more-special characters in the password?