Wrong User account for PDFCreator background task

Hi,
I am trying to use the lovely PDFCreator printer to use for automatic background jobs. I am using PDFCreator 1.7.3 for this.
A Windows Service is checking for incoming Office documents in a certain folder.
When
there is a file I start in C# a new process that calls the
PDFCreator.exe with the command line parameters /NoStart /ClearCache
/PF"<FileName>". The PDFCreator uses AuoSave feature to prevent
any popups.
I have created a batch file to do the same for testing
and it works...the file is created properly and I am able to see a
PDFCreator process in the Task Manager which is running under the
currently logged in user.
With the Windows Service I can first see a
PDFCreator process with the correct user but all in a sudden it starts
another PDFCreator process under SYSTEM user.
I tried to change anything in the C# project without any luck.
Did anyone of you tried the same and might help?
Or maybe I have to set the settings with AutoSave for the SYSTEM user? But how?

Thanks and best regards,
Thomas