Import from Jira - User mapping table is empty

When I try to Import Jira issues to GitLab, the “Jira-GitLab user mapping template” rests empty. No Jira users are displayed so that I could map them to GitLab Users.
When I ignore the problem and continue, all issues of the selected Jira project are imported, but the owner is defaulted to the user who performed the import.
Any ideas why the Jira users are not imported and the mapping template rests empty?