Those problematic attributes are additional_attributes created by me. The error shown is not correct, because those rows 250 and 265 have correct values, identical with the one set on the website for that attribute. For example, if I delete from CSV those problematic rows and try again, Magento will show the other 2 rows with the same problem. I had this problem even when I imported a brand new export with all the products from the website, but this time a different attribute was shown as problematic.
If I try to import some CSV without the additional_attributes column, everything works fine.