Console error - JQuery is not defined

Hi there,
I'm facing a strange problem and don't know how to deal with it

All my sites are suddenly showing in Firefox console "Jquery is not defined" when in the WP backoffice

Then lot of things in the backoffice are not working anymore...

I create a new site from scratch with WP 4.7.3 and WP4.7.5
And the console message appears at the install process
Nothing inside, new DB, Twenty Fifteen theme loaded and same problem
wp-config.php and .htaccess are cleans...

Some of my sites are hosted onto another server and problem is same
So seems that the server side is not involved

Would appreciate some help on this

Thanks