Dahmane – looks like the culprit is polylang plugin. When activated, the staff archive is seeing 404 error and the individual staff taxonomies are not displaying anything.
http://perfe-o.com/specialization/
None of the category archives are working except the default post categories. Can you quickly check with polylang support on why this happens? They should have encountered this for sure earlier and would know. Also, I removed the following code in header.php since it was raising error when polylang was deactivated –
<div id="header-language-switcher">
<?php pll_the_languages( array( 'display_names_as' => 'slug','show_names' => 0, 'show_flags' => 1, 'force_home' => 1 ) ); ?>
</div>
in header.php. Pls reinstate the same once you have the issue sorted out.
Sorry for the delay in replying – pls let us know if you need any information on our theme in case you hear back from polylang support.Thanks for your patience.