I had the same problem when I upgraded to 6.0 (A subsequent fresh install of 6.01 worked fine from the word go).
On my upgrade box I tried all of the posted advice and none worked. I solved it by deleting the table using the SQL admin tool (don't ask how, it was seat of the pants / Google search stuff - I know nothing about SQL).
I then re-ran the post upgrade scripts as per the forum posts and all worked.
Just to reiterate that although it worked for me, I have absolutely no idea what you could break (I think I lost my webmail settings, but I only had a few users on it at that point). Last resort
