Hello MassimoD,
I checked out all variables and I was unable to reproduce this on WordPress 4.4, 4.3 and 3.9 with the JustWrite theme.
The JustWrite Theme does (almost) everything according to standards from what I’ve seen so far. The only thing that’s outdated is the PHP7 compatibility (which isn’t released yet).
I assume it’s a dash added through the Tagline within /wp-admin/options-general.php
The Title Tag takes the Blog Tagline on the home page (this is per WordPress standards) and places it on the right per default.
As you’ve already noted, the disabling of Add site description (tagline) to the Title on the Home Page will remove the dash :).
If this doesn’t fix it, I’m flabbergasted and I must assume it’s another plugin messing this up.
Thanks and have a great day!