If a unbootable partitio is marked dos, it is added to lilo.conf automaticly. Lilo will not install, and you do not get a meaningfull error message. In fact, the lilo error message isn't even printed to the other VC's. Idealy, the installer should at least parse the error enough to tell you which partition is affected, or at least press <ALT><F3> for more info. Oh, and have it so you DONT have to run lilo manualy to figure out the problem.
Issue is resolved in 6.1 release.