Undefined offset: 0 in Drupal\csv_serialization\Encoder\CsvEncoder->extractHeaders()

I installed csv-export together with views_data_export (Drupal 8.5.3) . Blocked the export for all users that have not the right to administer users in views (not logged in and also logged in with no appropriate rights).

Then I tried to access the export with no appropriate rights (https://www.mydomain.org/user-data-export?_format=csv). The results: "The 'administer users' permission is required." I am not sure whether I find that good or if it tells the user too much. Most probably a Views thing.

From this I go back to the main domain (https://www.mydomain.org) and get a lot of error messages starting with:

Notice: Undefined offset: 0 in Drupal\csv_serialization\Encoder\CsvEncoder->extractHeaders() (line 176 of modules/contrib/csv_serialization/src/Encoder/CsvEncoder.php).
Drupal\csv_serialization\Encoder\CsvEncoder->extractHeaders(Array, Array) (Line: 143)
Drupal\csv_serialization\Encoder\CsvEncoder->encode(Array, 'csv', Array) (Line: 40)
Symfony\Component\Serializer\Encoder\ChainEncoder->encode(Array, 'csv', Array) (Line: 272)

No idea whether this is a views, csv_serialization or views_data_export . But the error messages are all from csv_serialization. After reload all error messages disappear.