I got the same error but for a completely different reason.
When I saved my Excel file as CSV, it appended hundreds of commas at the end of each line (even though there were no values in any of the cells). After I removed all the extraneous commas - after the last field - the import worked without any error.
It would help if MailWizz could provide proper error messages for specific cases like these (e.g. "empty tags or fieldnames on header line" and also "tried to import (null) values into non-existent fields").