Problem: Insert a watermark into an existing pdf document

the creation of pdf with watermarks from Word, Excel works OK. However, if I want to create a pdf with watermarks from a pdf document, I get the following error message:
Error NR.: 48 (file not found: gsdll32.dll)
If I open the PDFCreator printing monitor and print the document listetd in the print monitor again, the pdf is created correctly. If I leave now the PDFCreator printing monitor opened and try to created that PDF again, it works without problems.
As I said, the problem arises only if I want to insert a watermark into an existing pdf document. I inserted the watermark as follows:
PDFCreator - > Properties - > Document - > Stamp - > ...
Anybody know this problem? 
Greeting Stefan