My excuses, my fault. Strangely enough, neither XP nor OpenBSD were affected by my stupid mistake:
fdisk: /dev/discs/disc0/part1 1 7 Dell Utilitiy 2 8 900 HPFS/NTFS
^^^
3 891 1581 OpenBSD
^^^
4 1582 4864 Extended
OpenBSD's fdisk permitted the overlap; both XP and OpenBSD run well, but the Debian-Installer was annoyed.
After my corrections, the partitioning goes through. Sorry, again ! Uwe