Maybe your database is corrupted from your previous dnn version.
So you can check it and correcting it with the folow sql from Timo :
Dont forget to make a backup of your db before !
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http://support.dotnetnuke.com/issue/V...
You can check for pages with this error using this SQL script:
SELECT TabId, TabOrder, PortalId, TabName, IsVisible, ParentID, Level, IsDeleted, TabPath FROM Tabs WHERE
NOT ((LEN(tabpath)-LEN(REPLACE(tabpath, '//', ''))) / LEN('//')-1 = Level)
And you can correct them with this:
UPDATE Tabs SET
Level = (LEN(tabpath)-LEN(REPLACE(tabpath, '//', ''))) / LEN('//')-1