Access denied when performing batched export as user #1

This is a companion issue for https://www.drupal.org/project/views_data_export/issues/2800895 → .

Problem description is exactly as #2800895.

Trying to batch generate a data export file in CSV format results in a "access denied" error.
No CSV file can be downloaded.

The D6 version might be EOL, but it's an annoying fact that 'Views data export' is one of the few remaining options to upgrade at least some parts of D6 sites to newer Drupal versions.