Hi, the links are changed to be tracked, and probably you have the WP-Admin path locked with basic auth.
Just change the action link to the standard one on the Newsletter/Settings/Advanced, and the link is tracked in a different way.
Of course, resend a test newsletter to verify them.
If the metioned block is made with a rule inside the .htaccess, you can add an exception for the admin-ajax.php script, sometyhing like
<Files “wp-admin/admin-ajax.php”>
Require all granted
</Files>
Please don’t copy and paste the code above, retype it, and do not modify the htaccess if you are not used to.