Thanks for your answer. I will wait for the next official snapshot build an try it again.
> The issue has been reported on bugs[1] and later fixed[2]. The upgrade > procedure works fine for me by now. > > [1] http://marc.info/?l=openbsd-bugs&m=147402962517273&w=2 > [2] > http://cvsweb.openbsd.org/cgi-bin/cvsweb/src/distrib/miniroot/install.sub