I had this before and unchecked environment>scp/shell>lookup user groups.
Now when I go there I find this section greyed out, and 'lookup user groups' is ticked.
I can't untick it because it's greyed out.
Also the setting for the path to 'putty' keeps being reset to """"""C:\PuTTY\putty.exe""""""
and obviously doesn't work. Resetting the correct path does not 'stick', and the
above appears when WinSCP is restarted.