I already have both Service Packs, sorry, should have mentioned that.
A workaround was joining the cutted .avi-files, exporting them as one MPEG2-file, and using the .mpg-file for creating chapters and menu. In the end, the whole video doesn't need to be reencoded because of the "Do not encode compliant MPEG2-files" (or something). Still an uncomfortable feeling to make the project with a .mpg-file instead of raw material
but I guess it doens't matter at all ?
Now the next wedding-dvd(s) with sth. about 5 hours want to be created ^^ will try again if opening project with joined .avi-files will fail and I have to export the whole video...