When activating cache in "normal" mode, site is unaccessible because of call to undefined function drupal_add_js()

Thanks for reference. Please try the yui version 2.3. Please feed back.

I've just updated yui and yui_menu, enabled normal caching and everything seems to be fine :]

Fixed. Use yui 6.x-1.1-1 and yuimenu 6.x-1.1-1

I think this issue was fixed in a wrong way. You should not include common.inc inside a module file. That will lead to performance issues. Instead you should read suggested fix at http://api.drupal.org/api/function/hook_init/5.

One point is not clear in my mind. Should i remove the yui_init() function and move the 2 lines to yui_menu() like as follows ? Should it before the if($may_cache) line or after ? By the way many thanks for your advanced support.

function yui_menu($may_cache) {
  $skin = variable_get('yui_skin', 'yui-skin-sam');
  drupal_add_js("if (Drupal.jsEnabled) { $(document).ready(function() { $('body').addClass('$skin'); } ); };", "inline");  
  $items = array();
  if ($may_cache) {
    $items[] = array(
  ...
  ...

function yui_menu($may_cache) {
  $items = array();
  if ($may_cache) {
    $items[] = array(
  ...
  }
  else {
    $skin = variable_get('yui_skin', 'yui-skin-sam');
    drupal_add_js("if (Drupal.jsEnabled) { $(document).ready(function() { $('body').addClass('$skin'); } ); };", "inline"); 
  }
  ...

Fixed in 5.x-1.x-dev and will ready in next release.