When I try to load a disc project made with Magix Photo on CD/DVD version 6 in Magix Xtreme PhotoStory version 7 I get the error that files with extension *.HDP are not found. The files are there and in the correct directory. When I remove version 7 and install version 6 again everything works normal.
Is this correct?
It's certainly possible. Although I've heard that project files can be compatible between the program versions, it's not guaranteed to be compatible. I'd recommend finishing up that particular project on version 6, then moving on to version 7.