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Apparently, you have to update your cloned template set too, but just the set for NewBB.
So go to Sys Admin\Templates, delete the Newbb templates and then re-generate them.
I had the exact same problem and uninstalled it and have not yet had the courage to try again. There are many people who have had this problem (including me which is why I know the answer) - read the news article titled 'NewBB2 RC Release' currently listed on the home page near the bottom. You will get more info from there.
Good luck
Ted