I've never used the excel import but the conventional advice is to export 1 product to use as a template so you can see what format it expects.
Not sure whether that's the case, but line 1672 is looking to delete categories so you may need to step through the code to see how things look at this point:
//delete product categories
var deletedProductCategories = categories.Where(categoryId => !importedCategories.Contains(categoryId))
.Select(categoryId => product.ProductCategories.First(pc => pc.CategoryId == categoryId));
foreach (var deletedProductCategory in deletedProductCategories)