Maybe the sources aren't pointing at a server that's been updated.
I'd recommend reading this MDK10-CE to MDK10-Official update procedure, even if you're not going to update to Official, because it tells you how to find a working server, and goes through how to setup the sources.
http://anvil.webhop.org/mandrake/
You might try deleting and re-adding the secutity-updates source, and see if you can find the updates directory for MDK10-CE on a working mirror. It might take some searching because I think that the directories were swapped around somewhat when MDK10-official came out.