I had the same issue. Same error popped up in the Admin, on the already existing product pages.
I noticed that I didn't have that error message on a product create AFTER installing "Product Extra Fields".
The only solution I found was to get rid of all my products, uploaded all the products again and problem solved.
It works for me as I have about 80 products. But maybe Delia has a better idea...

I would pay to have someone upgrading "Product Extra Fields" to a version that would work with Easy Populate!!!

Damien