Update.
It seems this was a local problem after all.
Further runs with csup gave an error message related to
"/usr/sup/ports-all/checkouts.cvs:." ("Could not parse status record.
Delete it and try again."), so I deltedted the offendig line and ran
csup anew.
After that portupgrade worked again. Strange.
HTH
--
Regards,
Torfinn Ingolfsen