Load custom translation in custom plugin fails

I think Jacob has probably solved your problem in the comments.

However, to your question:

How to debug further this issue?

I would hook into the load_textdomain_mofile filter and check the fully built file paths WordPress is actually trying to read: Example:

<?php
function debug_missing_mofile( $mofile, $domain ){
   if( 'ausg' === $domain && ! is_readable($mofile) ){
       trigger_error('MO file not found at '.$mofile, E_USER_WARNING);
   }
   return $mofile;
}
add_filter('load_textdomain_mofile','debug_missing_mofile',999,2);

You should get two warnings. First for a missing file under WP_LANG_DIR, Second for a missing file that is probably your incorrect plugin path.

Notes:

• Up to you how you print the error. This example just raises a warning.
• Hooking with low priority to ensure other filters modifying paths have run.