sorry sorry sorry I must reply to myself
as i told the reason was within one click from the class page
seems that SHADING the forums would make them forever invisible
can anyone confirm that behaviour ? or is that some graphic special customization we have unpurposely got into ?
to revert the problem i called (from within browser)
class/configure-element?element_id=2368&op=shade
where 2368 is the forums element number i got from the portal layout customization page
seems this shading property of forums is persistent across removal and readdition of the portlet in the class
can anyone confirm ?