Now I have the following situation:
In our C# application, we used PDFCreator 0.9.3 and its COM interface to convert doc2pdf. But now, PDFCreator 0.9.5 is out and, having installed that, our app crashes at the following location of the constructor of my class:

public iPDFCreator()
{
m_pErr = new PDFCreator.clsPDFCreatorError();

m_pdfCreator = new PDFCreator.clsPDFCreator();
m_pdfCreator.eError += new PDFCreator.__clsPDFCreator_eErrorEventHandler(m_pdfCreator_eError);
m_pdfCreator.eReady += new PDFCreator.__clsPDFCreator_eReadyEventHandler(m_pdfCreator_eReady);

string parameters = "/NoProcessingAtStartup";
if (!m_pdfCreator.cStart(parameters, false))
{
m_pdfCreator.cClose();
}
}

Here's the exception that is thrown:

Schnittstelle nicht unterstützt
Exception Type = System.InvalidCastException
Source = mscorlib

Stack-Trace
===========
at System.Runtime.InteropServices.UCOMIConnectionPointContainer.FindConnectionPoint(Guid& riid, UCOMIConnectionPoint& ppCP)
at PDFCreator.__clsPDFCreator_EventProvider.Init()
at PDFCreator.__clsPDFCreator_EventProvider.add_eError(__clsPDFCreator_eErrorEventHandler )
at PDFCreator.clsPDFCreatorClass.add_eError(__clsPDFCreator_eErrorEventHandler )
at PrimaLib.IARLib.iPDFCreator..ctor() in ...