Down in the Hungarian forum, a member also had problems with Pepakura not opening SU-to-KMZ exports properly. It seems that the still available "Google Earth 4 (kmz" export from SU 6 works the best.
Note that SU 6 (Pro) is still available for download at Google and even if someone does not have a license for the pro version, after 8 hours it will gracefully degrade to the free version but that does have these kmz export options!