quickreport : EPrinter Error : Invalid Printer

I have a problem using the qr2.0 c version.

When I create a TQRReport, a Printer Error is thrown. I Use a German
Compiler, the Message means something like:
"EPrinter Error : Invalid Printer selected."

I tracked the Problem down to this :

- Normal Delphi printing works with the printers.printer.
(printer.printerindex := -1 works), the installed printers on the
machine do work and print, too.
- the qrPrinter in the unit qrprntr is not working, I tried to set the
printer by hand to -1 to reinitialize it :
if QRPrinter.Printers.Count > 0 then
    QRPrinter.PrinterIndex := -1;
this line throws the excpetion "eprintererror : Invalid printer
selected"
- the quickreports did function at the beginning (for a few weeks), we
didn't change the configuration and now they obscurely don't work
anymore, just throw the mentioned error.

can you, dear reader, help us or had a similar problem?
we are desperate because the source of quickreport
isn't public like the delphi source is and therefore we can't track the
error in unit qrprntr