lea_

Yes I did

– re-uploading all files & folders – except the wp-content folder and the wp-config.php & root .htaccess files – from a fresh download of WordPress. Make sure that you delete the old copies of files & folder before uploading the new ones.
=> DONE

Nothing new ??
Still missing some admin pages like widgets, and no update page on the dashboard menu … I’m crying ^^

switching to default theme, disable pluggins and upgrade manually : already DONE.

re-uploadingfiles : do you mean I must first delete those on my ftp or I can overwrite : I already upload latest WP files on my ftp files, with overwriting option with the same actual issue.