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:
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.