It would still be good to find out what the complete SQL error message is - from you error.log
Now - what happens when you try to create a new top level category - are there no errors at all?
Have you used the distributed language file or made modifications?
This problem is hard to understand since your not getting an error when you add the category - your just not seeing it listed when you go to add a sub category. I assume it also does not appear in the display - file listing or when you try to add a file.
I may need admin access to your geeklog site to see this as I've never heard of this one before. If your interested - create an account for me and send me the details.
Blaine