Thank you for your quick reply and hints.
There is indeed an upper filter driver defined. It’s the GEARAspiWDM driver 2.2.0.1 of GEAR Software Inc. After renaming the registry key UpperFilters into _UpperFilters followed by a reboot WinCDEmu works again like charme. I know the above solution is just a hack as one can have several upper filter drivers, etc.
My initial assumption was right because this filter driver was installed together with the iTunes (Norton Ghost and Norton 360 also use this driver according to the GEAR Software Wiki).
Starting iTunes with the modified registry I get an error message box that burning/ripping CDs is no longer possible and I have to re-install iTunes. Thankfully the program continues to start after clicking OK. As I consider this apple software as a pain in the ass with all of the useless services running in background after installation and just use it to synchronize mp3s, apps, etc. I can live with this. Of course it would be great if WinCDEmu would be able to work even if this filter is installed.