If the persons in Marketo are linked to contacts and accounts in your CRM, the company fields are locked and cannot be updated.
But I was under the souvenir that in this case, the import would update the contact fields and let alone the account ones, but would not reject the import.
Can you check whether you are in such a contact/account situation ? (look at the SFDC type field).
Many thanks for your answer.
Yes, that's correct, the contacts affected were all linked to contacts and accounts in our CRM and wouldn't update until I took out all company related fields from the spreadsheet. After that they updated.
Again many thanks.