New Post: Ciritical error when adding product to checkout or going to the checkout page

You might need that discounts calculator task running:
https://nbstore.codeplex.com/wikipage?title=Scheduled%20tasks

The other error still seems likely to be caused by malformed xml. Have you got specific templates applied to specific categories or does the whole store run the same templates. Maybe you can export the product catalogue and then compare the xml of a good product with a faulty one.
Hard to say really but you're obviously on the right track
rob