The solution was easy, after I noticed that they haven't installed any printer-driver on the system:
include RpDevice in the uses and then you can check before the rave executes the existence of any printer:
if RpDev.InvalidPrinter then
showmessage('no printer'); -> and you should disable the execute of RvProject in the application
The older way with printers count can make trouble with network printers and isn't recommended (RPDev.Printers.Count).
After installation of a small printer driver (I prefer the HP LJ4 for things like this) the application creates reports with Rave on the customers server without any problems....