When you say "Kentico 8.3" do you mean "8.0.3" or did you type it wrong, since there isn't an 8.3
Next, when doing your upgrade, what version did you start with? It's important when going through major versions (say 8.0 to 8.1, 8.1 to 8.2) to load the site full in between your major upgrades, if you try to do one after the other without allowing the site to load fully after each one, key code is not run that finishes the upgrade and you will end up with a failed upgrade.