I was able to find out more precisely what went wrong. I first tried booting into Windows, which executed "set root=(hd0,0)". That's the wrong partition, it should have been (hd0,1). So it failed booting, returning me to the menu. Then I selected debian, but it couldn't find the kernel because the root was set to (hd0,0). To protect against this the menu-entry for debian should include the set root command. Alexander