Rick has confirmed that there were no changes to the database between 2.0 and 2.1 so perhaps your database has corrupted?
I upgraded recently from 2.0 to 2.1, by downloading the upgrade package (i.e. not the full install). All I did was to copy and replace the changed files from that upgrade over the top of my existing tiki installation. No database scripts, no installer initializations, and it worked fine, without any errors of any kind.
Maybe you could backup your database and try to go back to 2.0 and see if you still get errors. Then try the upgrade again if not.