Ummm, but clearly it affects order totals which means when you administer orders, errors could indeed show up in the admin -- otherwise you wouldn't get this error message, right???
The "call to an undefined function" error says that the issue is possibly with your Rewards Points module.. I would take a look at your Rewards Points module and make sure it's installed correctly..
So going back to my original response to you requesting more information.. you still haven't answered:
What other modules do you have installed??
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