A bug has been found in FTPSync, which may cause it to reject INCREMENTAL command line parameter. If you are using /INCREMENTAL command line parameter, like:
FTPSync myinifile /INCREMENTAL
you should instead call FTPSync without it:
FTPSync myinifile
because this is default parameter and is not needed. This will work always and prevent you from running into "Illegal parameter" error.
This issue has been resolved in FTPSync 2.8b.