Hello, coming again for this issue as it's still not resolved.
My Website Address & Site Address URLs are properly set.
The issue exists on the front-end as it seems on each page load AcyMailing does an AJAX call for ajaxGetEnqueuedMessages.
So the admin-ajax.php file is located within the wp-admin folder and it should be called from there. I am not sure why you're calling it relatively to the path you are on the site.
Why not load the admin-ajax.php file via
admin_url('admin-ajax.php')? This should resolve these issues