Hi Odedta and Eschwartz,
Thanks for taking the time to answer me with suggestions ! Much appreciated !
(and sorry for the late answer due to the end of year celebrations)
>>try changing the id of the nav.xhtml to toc maybe that confuses the validator...
I had actually tried that already, and got another error - if I remember well, the Validator then told me that my nav must "supply a landmarks".
Before getting Eschwartz's answer, I rebuilt all my files from scratch, using a template from a commercially released epub, and this time it worked well.
I'm pretty sure Eschwartz was right (>>I don't see <nav epub:type="toc" id="toc"> in your nav file ), this line is just what was missing and needed.
Those error messages are way too cryptic
thanks again !