Undefined index: #attributes
You need to accept the $form (and $form_state) parameters as part of the function signature, and not overwrite the form, rather add to it:

function gestionvideos_configuration($form, &amp;$form_state) {
  $form['#submit'][] = 'gestionvideos_configuration_submit';

  $form['config'] = array(
    '#type' =&gt; 'fieldset',
    '#title' =&gt; t('Administration du plugin VOD'),
    '#description' =&gt; t('Pour fonctionner, le plugin a besoin de s\'interfacer avec votre compte VOD infomaniak.&lt;br/&gt;
                Pour des raisons de securites, il est fortement conseille de creer un nouvel utilisateur dedie dans votre admin infomaniak avec uniquement des droits restreints sur l\'API.&lt;br/&gt;
                Pour plus d\'information, veuillez vous rendre dans la partie "Configuration -&gt; Api &amp; Callback" de votre administration VOD.  
    '),
    '#default_value' =&gt; variable_get('config', ''),
  );

  return system_settings_form($form);
}


Choosing a different name than 'config' for your variable might also be wise. It's always preferable to provide a pseudo-namespace to variables as a prefix, usually the module name (e.g. gestionvideos_config). Don't forget to add the default value as in the above example as well, otherwise next time you go to the form it will have forgotten your config option.

If the above doesn't work for whatever reason, then the bug is somewhere else; try systematically disabling custom/contrib modules until you find the culprit.