Upgrade to 6-2-7 problems

Just tried to upgrade from 6-1-15 to 6-2-7 by replacing the core folder and got some problems.

Trying to view a page more than one level down in the navigation resulted in the following vague error:

There was a problem with that last request Please push "back" on your browser or go back home

Clicking on a navigation item in the site tree resulted in the following error (clicking on html pages worked fine):

Element NAVTYPE is undefined in STOBJ.

Has anyone experienced this problem or have any ideas what might be the cause?

I realise I'm not providing a lot of information here but this was in our live environment and I needed to revert back as quickly as possible.

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