Thanks for the details; I overwrote the mbr using dd of=/dev/rsd0c if=/usr/mdec/mbr bs=512 count=1
and then used fdisk to reinstall the OpenBSD partition to the values I used before - and now the system can boot from the ssd. Thank you very much for the help! -- Address is valid for this mailing list only, please do not reply to it direcly, but to the list.